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Epidemic Influenza and Vitamin D

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									Epidemiology and Infection
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Epidemic influenza and vitamin D
J. J. CANNELL, R. VIETH, J. C. UMHAU, M. F. HOLICK, W. B. GRANT, S. MADRONICH, C. F. GARLAND and E. 
GIOVANNUCCI

Epidemiology and Infection / Volume 134 / Issue 06 / December 2006, pp 1129 ­ 1140
DOI: 10.1017/S0950268806007175, Published online: 07 September 2006

Link to this article: http://journals.cambridge.org/abstract_S0950268806007175

How to cite this article:
J. J. CANNELL, R. VIETH, J. C. UMHAU, M. F. HOLICK, W. B. GRANT, S. MADRONICH, C. F. GARLAND and E. 
GIOVANNUCCI (2006). Epidemic influenza and vitamin D. Epidemiology and Infection, 134, pp 1129­1140 doi:10.1017/
S0950268806007175

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Epidemiol. Infect. (2006), 134, 1129–1140. f 2006 Cambridge University Press
doi:10.1017/S0950268806007175 Printed in the United Kingdom



REVIEW ARTICLE
Epidemic influenza and vitamin D



J. J. C A NN E L L 1*, R. V IE T H 2, J. C. UM H A U 3, M. F. H O L IC K 4, W. B. GR A NT 5,
S. M A D R O N I C H 6, C. F. G A R LA ND 7 A N D E. G I O V A N N U C C I 8
1
  Atascadero State Hospital, 10333 El Camino Real, Atascadero, CA, USA
2
  Mount Sinai Hospital, Pathology and Laboratory Medicine, Department of Medicine, Toronto, Ontario,
Canada
3
  Laboratory of Clinical and Translational Studies, National Institute on Alcohol Abuse and Alcoholism,
National Institutes of Health, Bethesda, MD
4
  Departments of Medicine and Physiology, Boston University School of Medicine, Boston, MA, USA
5
  SUNARC, San Francisco, CA, USA
6
  Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
7
  Department of Family and Preventive Medicine, University of California San Diego, La Jolla, CA, USA
8
  Departments of Nutrition and Epidemiology, Harvard School of Public Health, Boston, MA, USA

(Accepted 5 August 2006, first published online 7 September 2006)

SUMMARY
In 1981, R. Edgar Hope-Simpson proposed that a ‘seasonal stimulus ’ intimately associated with
solar radiation explained the remarkable seasonality of epidemic influenza. Solar radiation triggers
robust seasonal vitamin D production in the skin ; vitamin D deficiency is common in the winter,
and activated vitamin D, 1,25(OH)2D, a steroid hormone, has profound effects on human
immunity. 1,25(OH)2D acts as an immune system modulator, preventing excessive expression
of inflammatory cytokines and increasing the ‘oxidative burst ’ potential of macrophages. Perhaps
most importantly, it dramatically stimulates the expression of potent anti-microbial peptides,
which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the
respiratory tract where they play a major role in protecting the lung from infection. Volunteers
inoculated with live attenuated influenza virus are more likely to develop fever and serological
evidence of an immune response in the winter. Vitamin D deficiency predisposes children to
respiratory infections. Ultraviolet radiation (either from artificial sources or from sunlight) reduces
the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D). An
interventional study showed that vitamin D reduces the incidence of respiratory infections in
children. We conclude that vitamin D, or lack of it, may be Hope-Simpson’s ‘seasonal stimulus ’.


INTRODUCTION                                                        … the characteristic microbe of a disease might be a
                                                                    symptom instead of a cause.
Whoever wishes to investigate medicine properly should                                                George Bernard Shaw
proceed thus : in the first place to consider the seasons of the            (Preface on Doctors, The Doctor’s Dilemma, 1911)
year …
                                                   Hippocrates
                                                                    Perhaps the most mysterious feature of epidemic
                                                (circa 400 B.C.)
                                                                    influenza is its remarkable and recurrent seasonality –
                                                                    wintertime surfeit and summertime scarcity – a
* Author for correspondence: Dr J. J. Cannell, Atascadero State
Hospital, 10333 El Camino Real, Atascadero, CA 93422, USA.
                                                                    feature first explored in detail by R. Edgar Hope-
(Email: jcannell@dmhash.state.ca.us)                                Simpson, the British general practitioner and
1130                                                              J. J. Cannell and others

                                                            Latitude zone         Quarter 1    Quarter 2            Quarter 3          Quarter 4                                    1200
 Monthly percentage of total epidemic months in each zone                        Jan·Feb·Mar Apr·May·Jun Jul·Aug·Sep Oct·Nov·Dec
                                                            N. temperate 30                                                                                                         1000




                                                                                                                                                     Influenza case rates/100 000
                                                            N. 30° +        20
                                                                                                                                                                                     800
                                                                            10
                                                                             0
                                                                                                                                                                                     600
                                                                            20                             tical solar radi
                                                            N. tropical                                 ver                ati
                                                                                                     of     23·5° N           o




                                                                                                th
                                                                            10




                                                                                                                              n
                                                            N. 0–29°

                                                                                              Pa
                                                                                                                                                                                     400
                                                                             0                 0°           Equator               0°
                                                                            20
                                                            S. tropical                                                                                                              200
                                                                            10
                                                            S. 0–29°         0 23·5° S                                                    23·5° S                                      0
                                                            S. temperate 40                                                                                                                S O ND J F MAM J J A S O N D J F MA
                                                            S. 30° +     30                                                                                                                 1968          1969           1970
                                                                            20
                                                                                                                                                    Fig. 2. Weekly consultation rates for illnesses diagnosed
                                                                            10
                                                                                                                                                    clinically as influenza or influenza-like, calculated from re-
                                                                             0                                                                      turns to the General Practice Research Unit of the Royal
Fig. 1. The seasonal and latitudinal distribution of                                                                                                College of General Practitioners from about 40 general
outbreaks of type A influenza in the world, 1964–1975,                                                                                               practices in various parts of England, Scotland and Wales,
summarized from the Weekly Epidemiological Record of the                                                                                            serving a population of about 150 000 persons, 1968–1970.
World Health Organization into major zones. The diagrams                                                                                            (Reproduced/amended with permission, BMJ Publishing
show for each calendar month the percentage of each zone’s                                                                                          Group, Miller et al.)
total outbreaks. In both north and south temperate zones
the epidemics are distributed around the local midwinter,                                                                                           Thus, he hypothesized that solar radiation produced
whereas the tropical zones show a transition, each approxi-
                                                                                                                                                    a ‘seasonal stimulus ’ that profoundly affected the
mating towards the distribution of its own temperate zone.
The curve indicates the ‘midsummer ’ path taken annually                                                                                            pathogenesis of influenza A – but he had no idea of
by vertical solar radiation. The ‘epidemic path’ seems to                                                                                           the mechanism. However, Hope-Simpson believed
parallel it, but to lag 6 months behind it. (Reproduced with                                                                                        epidemiologists would eventually succeed with ‘the
permission, Cambridge University Press, Hope-Simpson,                                                                                               task of identifying the chain of intermediate mechan-
1981.)                                                                                                                                              isms through which the prime cause (the variation
                                                                                                                                                    in solar radiation) is operating its seasonal influence’
self-educated epidemiologist. After his celebrated dis-                                                                                             [3, p. 87].
coveries of the cause of shingles [1] and the latency of                                                                                               Although serological and culture evidence of influ-
varicella [2], Hope-Simpson dedicated much of the rest                                                                                              enza infection has been documented in the summer,
of his working life to the epidemiology of influenza.                                                                                                it seldom causes community outbreaks in summer
He believed that discovering the cause of influenza’s                                                                                                [5–7]. About 2 % of persons continuously surveyed
seasonality would ‘provide the key to understanding                                                                                                 seroconvert during periods when clinical influenza is
most of the influenzal problems confronting us ’ [3].                                                                                                not recognized [8]. In spite of being in the population
   Hope-Simpson was the first to document that                                                                                                       year-round, epidemics in temperate latitudes usually
influenza A epidemics in temperate latitudes peak in                                                                                                 peak in winter [9–13]. Furthermore, Hope-Simpson
the month following the winter solstice (Fig. 1). In                                                                                                noted that ‘ epidemics of influenza often occur con-
both hemispheres, influenza rates rise significantly                                                                                                  temporaneously at the same latitude even in localities
for about 2 months on either side of its peak.                                                                                                      widely separated by longitude ’ [4, p. 43]. He noted
                                                                                                                                                    influenza would abruptly attack 15 % or more of the
Outbreaks are globally ubiquitous and epidemic loci move
smoothly to and fro across the surface of the earth almost                                                                                          population around the winter solstice but virtually
every year in a sinuous curve that runs parallel with the                                                                                           disappear in the sunny months despite a wealth of
midsummer curve of vertical solar radiation, but lags about                                                                                         potential victims lacking virus-specific antibodies.
six months behind it … Latitude alone broadly determines                                                                                               Hope-Simpson saw solar radiation as a stronger
the timing of the epidemics in the annual cycle, a relation-
                                                                                                                                                    predictor of influenza epidemics than the presence
ship that suggests a rather direct effect of some component
of solar radiation acting positively or negatively upon the                                                                                         of virus-specific antibodies. For example, Miller et al.
virus, the human host, or their interaction … The nature of                                                                                         [14] reported that the Hong Kong virus was first
the seasonal stimulus remains undiscovered [4].                                                                                                     isolated in Britain in August 1968 but it did not
                                                                                      Epidemic influenza and vitamin D                1131

cause significant summertime illness despite being a                            35
new antigenic variant in a non-immune population
(Fig. 2). However, clinical case rates increased in                            30




                                                             25(OH)D (ng/ml)
intensity as the sun became progressively lower in                             25
the sky each day (autumn), waiting until the winter
solstice of 1968 before the first community outbreaks                           20
appeared. Influenza case rates peaked for several
                                                                               15
months but waned as the sun rose higher in the sky
each day (spring). Predictably, influenza virtually                             10
                                                                                Aug. Sep. Oct.Nov.Dec. Jan. Feb. Mar.Apr. MayJune JulyAug.
ceased following the summer solstice. Clinical case
                                                                                                          Month
rates for Hong Kong influenza increased from
September 1969, only to explode again in the days           Fig. 3. Seasonal variation of 25(OH)D levels in a popu-
preceding the winter solstice, even though a much           lation-based sample of inhabitants of a small southern
                                                            German town, aged 50–80 years. (Reproduced/amended
higher proportion of the British population had virus-      with kind permission of Springer Science and Business
specific antibodies at the beginning of the lethal           Media, Scharla, S.H., 1998.)
second wave than they did at the beginning of its
less lethal first wave.                                      with profound effects on human immunity whose
   Hope-Simpson also observed that influenza out-            substrate levels reach their nadir during influenza
breaks in the tropics, where solar UV radiation is less     season but peak when influenza is rare (Fig. 3) [18, 19].
seasonal, are also much less seasonal, but are gener-          Cholecalciferol (vitamin D) is a prehormone
ally more severe when solar radiation is impaired (the      normally made in the skin during sunny months
rainy season) – observations recently confirmed [15].        when UVB radiation triggers the conversion of 7-
This intimate association with sunlight led the natu-       dehydrocholesterol in the skin into vitamin D [20].
ralist in Hope-Simpson to see influenza as a winter          The liver converts vitamin D into 25-hydroxyvitamin
‘crop, and, as with other crops, some years are good        D [25(OH)D] and then cells all over the body
influenza years, and other years produce a poor crop         convert 25(OH)D to 1,25-dihydroxyvitamin D
of influenza cases ’ [3, p. 92].                             [1,25(OH)2D] – a potent steroid hormone. Locally
   Influenza is but one of several respiratory viral         produced 1,25(OH)2D performs autocrine and para-
pathogens that show a distinct predilection for             crine functions in a wide variety of tissues, including
infecting us in the wintertime. Noah found that in          the immune system. Local tissue levels of 1,25(OH)2D
England and Wales respiratory syncytial virus and           are dependent on available serum substrate
parainfluenza 1 and 2 display marked wintertime              [25(OH)D]. Dangers of vitamin D deficiency may
excess [16]. More than 200 viruses cause the common         include more than just low 25(OH)D levels. Vieth has
cold, which, as the name implies, also shows a distinct     proposed that progressively falling serum levels of
wintertime excess [17]. However, in clinical practice,      25(OH)D (as occurs in the autumn), may trigger
and in much published research, specific identifi-            intracellular deficiencies of 1,25(OH)2D, despite
cation of respiratory viral infections is frequently        apparently adequate serum levels of 25(OH)D and
absent. With full appreciation of its inherent limi-        1,25(OH)2D [21].
tations, we will use the term viral respiratory infection      Many distinctive features of the biology, physi-
in this review, unless the literature cited was more        ology, and epidemiology of vitamin D point to it
specific.                                                    as a likely candidate for Hope-Simpson’s ‘ seasonal
                                                            stimulus ’.
                                                            (1) Vitamin D has profound and multiple effects on
The seasonal stimulus ?
                                                                human immunity [22, 23].
As Hope-Simpson pointed out, solar radiation may            (2) Inadequate vitamin D nutrition is endemic among
be affecting the ‘virus, the human host, or their                the elderly in the winter [24–26].
interaction … ’. That is, he theorized that humans          (3) Serum levels of 25(OH)D are low in many
might have a physiological system directly dependent            people of all ages who live at temperate latitudes,
on solar radiation that improves innate immunity                especially in the winter [20].
around the summer solstice but impairs it in the            (4) Humans acquire most of their vitamin D
winter. There is a seasonal steroid hormone system              from casual sun exposure, and to a degree that
1132     J. J. Cannell and others

    is a function of skin surface area exposed             cathelicidins, directly destroy invading microorgan-
    [27, 28].                                              isms [46]. AMP display broad-spectrum antimicrobial
(5) The elderly only make about 25 % of the vitamin        activity, including antiviral activity, and have been
    D as 20-year-olds do after exposure to the same        shown to inactivate the influenza virus [47–49]. Not
    amount of sunlight [29].                               only do neutrophils, macrophages, and natural killer
(6) Seasonal variations – and vitamin D deficiency –        cells secrete AMP, but epithelial cells lining the upper
    occur in both subtropical and tropical latitudes       and lower respiratory tract secrete them as well, where
    [30, 31].                                              they play a major role in pulmonary defence [50, 51].
(7) Routine daily supplementation with 400 IU of
    vitamin D does not prevent wintertime insuf-
                                                           Influenza and solar radiation
    ficiency [32].
                                                           In spite of people congregating on cruise ships, air-
                                                           planes, nursing homes, factories, offices, subways,
Mechanism of action of vitamin D
                                                           hospitals, etc., summertime outbreaks and the spread
The pathology of influenza involves a complex               of influenza A are rare [52, 53]. Curwen found a
interaction between the virus, acquired immunity, and      strong inverse correlation between the incidence of
innate immunity. Macrophages rapidly release cyto-         influenza and temperature in England and Wales [54],
kines into infected respiratory tissue while virucidal     temperature being strongly associated with solar
antimicrobial peptides attempt to prevent viral repli-     radiation [55]. He found average monthly tempera-
cation [33]. The release of proinflammatory cytokines,      ture dropped below around 7 xC during the influenza
as much as the virulence of the virus, may determine       season. It is of interest that no vitamin D is made in
the clinical phenotype of influenza infection. Recent       the skin at latitude 52x N (the latitude of London)
research confirms that the clinical phenotype of            from about October to March because atmospheric
influenza correlates well with amount of cytokines          ozone easily filters out UVB radiation unless the
released [34, 35]. Furthermore, the severity of the        sun is high enough in the sky [56].
illness induced by genetically reproduced 1918 influ-          Annual all-cause mortality peaks in the months
enza virus also correlates with the ability of the virus   following the winter solstice and most excess winter-
to induce macrophage production of cytokines [36].         time mortality is in the elderly, due to both influenza
In avian influenza, the innate cytokine immune re-          and cardiac disease ; some believe influenza explains
sponse can be overwhelming; levels of such cytokines       all the significant wintertime increase in cardiac
are significantly higher in those with a fatal outcome      mortality [57]. The average excess winter mortality in
[37, 38].                                                  Great Britain alone is 30 000 persons per year [58],
   Recently, vitamin D has been found to modulate          and is inversely related to hours of sunlight with 2.9 %
macrophages’ response, preventing them from re-            lower odds for every additional hour of sunshine ;
leasing too many inflammatory cytokines and chemo-          mortality from respiratory disease showed the greatest
kines [39, 40]. Vitamin D deficiency also impairs           sunshine benefit.
the ability of macrophages to mature, to produce              If vitamin D is Hope-Simpson’s ‘seasonal stimu-
macrophage-specific surface antigens, to produce the        lus ’, then countries with low 25(OH)D levels and
lysosomal enzyme acid phosphatase, and to secrete          marked wintertime troughs should have higher ex-
H2O2, a function integral to their antimicrobial           cess wintertime mortality than do countries with
function [41, 42]. The same authors found that the         high 25(OH)D levels and little seasonal variation.
addition of 1,25(OH)2D increased expression of             For example, Norway has the highest 25(OH)D
macrophage-specific surface antigens and the lyso-          levels in Europe (thought to be due to its high
somal enzyme acid phosphatase while stimulating            year-round consumption of fish and cod liver oil)
their ‘oxidative burst ’ function.                         [59]. Levels of 25(OH)D in Scandinavia display
   Perhaps most importantly, three independent re-         the least seasonal variation in Europe ; indeed there
search groups have recently shown that 1,25(OH)2D          is virtually no 25(OH)D seasonal variation among
dramatically stimulates genetic expression of anti-        the elderly in Scandinavia [60]. On the other hand, the
microbial peptides (AMP) in human monocytes,               elderly in Great Britain have low 25(OH)D levels
neutrophils, and other human cell lines [43–45].           and such deficiencies are much more common
These endogenous antibiotics, such as defensins and        during the influenza season [61]. Excess wintertime
                                                                     Epidemic influenza and vitamin D        1133

mortality is twice as high in Great Britain as in          mortality from combined pneumonia and influenza
Norway [62].                                               deaths in the United States was higher for African-
   Global weather changes are associated with El           Americans than for whites in 2000 (10 % excess), 2001
Nino/Southern Oscillation (ENSO) [63]. Viboud et al.
    ˜                                                      (11 % excess), and 2002 (6 % excess) [73–75]. The
found an average of 3.7 million influenza cases in          same statistics show African-Americans have a much
France during the 10 cold phases of ENSO but only          higher age-adjusted mortality from heart disease,
1.8 million cases during the eight warm phases [64].       and a significant percentage of those who die from
The same authors reported that cold ENSO phases            influenza are reported to have suffered a cardiac death
are associated with colder temperatures in Europe.         [76, 77]. Furthermore, black children continue to
Colder temperatures should lower mean serum                have twice the pneumonia mortality of white
population 25(OH)D levels by lessening outdoor             children [78]. While some of these racial disparities
activity and necessitating more clothes when out-          may be due to socioeconomic factors, racial differ-
doors. Ebi et al. studied six Californian counties and     ences in 25(OH)D levels may also be important.
found that hospitalizations for viral pneumonia
peaked around the winter solstice in all six counties
                                                           Attenuated influenza virus, the effect of season
[65]. They also found hospitalizations increased
30–50 % for every 5 xF (3 xC) decrease in minimum          If vitamin D is Hope-Simpson’s ‘seasonal stimulus ’,
temperatures in four counties and increased 25–40 %        then humans inoculated with attenuated viruses
for every 5 xF (3 xC) decrease in maximum tempera-         during the summer [when 25(OH)D levels peak]
tures in the other two.                                    should show less evidence of infection than those in-
   Hope-Simpson was the first to note an association        oculated in winter. Shadrin et al. inoculated 834 non-
between severe influenza epidemics and solar flare           immune males (age 16–18 years) with live attenuated
activity [66]. In 1990, Hoyle and Wickramasinghe           influenza virus (B/Dushabbe/66 and B/Leningrad/2/
confirmed the association but von Alvensleben dis-          67) in St Petersburg (62x N) and Krasnodar, Russia
puted it [67, 68]. Horgan [69] promptly derided the        (45x N), during different seasons of the year, com-
observations, connecting them to viral invasions from      paring them to 414 vehicle placebo controls [79]. In
outer space, a theory Hope-Simpson dismissed in his        St Petersburg, they found that the attenuated virus
1992 book [3]. Since the controversy, science has          was about eight times more likely to cause physical
learned that solar flare activity increases high-altitude   evidence of infection (fever) in the winter than
ozone, which, in turn, absorbs more UVB radiation          the summer (6.7% vs. 0.8%). In Krasnodar, 8% of
thereby decreasing surface UVB [70]. Thus, para-           inoculated subjects developed a fever from the virus
doxically, heightened solar activity reduces surface       in January, but only 0.1% did so in May.
UVB ; presumably, average 25(OH)D levels would be             Zykov and Sosunov found that fever after
lower as well. Rozema et al. estimated the variations      inoculation with attenuated H3N2 (221 subjects) was
in surface UVB radiation due to the solar flare activity    twice as likely in February (10.7%) as in June (5 %),
over the last 300 years and estimated that, beginning      compared to vehicle placebo controls [80]. They also
in the eighteenth century, ‘the dose of surface UV-B       confirmed that seroconversion varied by season, with
should be (about) 4 % to 13 % lower at maxima of           the lowest rate of antibody formation in summer.
the 11-year solar cycle ’ [71]. Although modest, such      When they attempted to recover the virus 48–72 h
reoccurring decreases in UVB radiation should trigger      after inoculation, they found subjects were more
reductions in average 25(OH)D levels, which, in turn,      likely to shed the virus in December (40 %) than in
could trigger nonlinear factors related to influenza        September (16 %), and the quantity of virus shed was
infectivity.                                               significantly lower in summer than winter.
   Melanin retards the ability of sunlight to trigger
vitamin D production in African-Americans’ skin so
                                                           Vitamin D deficiency and viral respiratory infections
they have much lower 25(OH)D levels than do whites
[72]. If vitamin D is Hope-Simpson’s ‘ seasonal            If vitamin D is Hope-Simpson’s ‘seasonal stimulus ’,
stimulus ’ then African-Americans should have a            then vitamin D deficiency should predispose patients
higher incidence of influenza and higher age-adjusted       to respiratory infections. Rickets is the classic
influenza mortality than do whites. Although we             vitamin D-deficient disease of childhood and a
could find no racial incidence data, the age-adjusted       long-standing association exists between rickets and
1134     J. J. Cannell and others

respiratory infection [81–88]. Mechanical impair-            large amounts of vitamin D. All five cod liver oil
ment of pulmonary function due to clinical rickets is        studies listed by Semba showed it reduced the inci-
widely thought to explain the association. However,          dence of respiratory infections. Two controlled
Wayse et al. recently compared 80 non-rachitic               studies in the 1930s found similar results : the first
children with lower respiratory infections to healthy        found cod liver oil given to 185 adults for 4 months
controls and found children with 25(OH)D levels              reduced colds by 50 % ; in the second study it reduced
<10 ng/ml were 11 times more likely to be infected           industrial absenteeism due to respiratory infections in
[89]. This discovery makes it likely that it is vitamin      1561 adults by 30% [98, 99]. In 2004, Linday et al.
D deficiency per se, and not mechanical impairment            reported that 600–700 IU of vitamin D, given as cod
of pulmonary function, that explains the long-               liver oil and a multivitamin, significantly reduced the
standing association of rickets with pulmonary               mean number of upper respiratory tract visits over
infection.                                                   time when given to 47 young (mean age 2 years)
                                                             New York City children from late autumn to early
                                                             May, whereas in a medical record control site group,
UV radiation and viral respiratory infections
                                                             no decrease occurred over time [100]. Assuming the
It is generally accepted that erythemal doses of UV          average 2-year-old weighs 13 kg, an equivalent dose in
radiation (UVR), which contains both UVB and UVA             a 70 kg adult would be about 3500 IU/day.
radiation, suppress human immune function [90, 91].
However, Termorshuizen et al. recently reviewed the          Intervention with vitamin D. We are aware of only
literature on immune function and UVR, concluding            one modern paper that directly examined the relation-
it is dangerous to assume that such suppression will         ship between vitamin D and respiratory infections.
result in an increased incidence of infectious disease       Rehman, in a letter, reported giving 60 000 IU of
[92]. Furthermore, sub-erythemal doses of UVR,               vitamin D a week and 650 mg of calcium daily for
unlike erythemal doses, actually improve phagocytic          6 weeks to 27 non-rachitic children (aged 3–12 years)
activity in human volunteers. For example, Krause            with elevated alkaline phosphatases who were also
et al. reported that a 6- to 8-week course of sub-           suffering from frequent childhood infections, mostly
erythemal doses of UVR doubled the phagocytic                respiratory infections [101]. He compared them to
activity in 21 children with recurrent respiratory           20 age- and-sex matched control children who had not
tract infections [93]. Likewise, Csato et al. found five      had more than one infectious episode per child during
sub-erythemal doses of UVR increased polymorpho-             the previous 6 months. During the 6 months of ob-
nuclear chemotaxis in normal volunteers [94].                servation after treatment, no difference was observed
   In 1990, Gigineishvili et al. administered sub-           in the frequency of infection between the test and
erythemal courses of UVR twice a year for 3 years to         control groups of children. In fact, Rehman reported,
410 teenage Russian athletes and compared them               ‘ no recurrences were reported for a period of six
to 446 non-irradiated athletes [95]. The non-UVR             months ’, in the treated children.
controls had 50 % more respiratory viral infections,
300 % more days of absences and 30 % longer                  DISCUSSION
duration of illness than did the UVR subjects. The
irradiated subjects also had significant increases in         The most common explanation for the seasonality of
salivary IgA, IgG and IgM compared to controls. In           viral respiratory infections is that humans congregate
2004, Termorshuizen et al. found that parents of             indoors in the winter, thus increasing the chance for
Dutch children with the least sun exposure were twice        contagion. However, as Sir Christopher Andrewes
as likely to report that their child developed a cough,      pointed out, people also congregate indoors during
and were three times as likely to report their child had     the summer [102].
a runny nose, compared to children with the most             Many people regard (crowding) as the likeliest ‘ winter fac-
sun-exposure [96].                                           tor ’ to explain the facts (wintertime excess of respiratory
                                                             infections). I have always had doubts about this. Indoor
                                                             workers in towns spend their working hours in much the
Cod liver oil and viral respiratory infections               same way winter and summer; they are cheek-by-jowl in
                                                             their offices or at the factory bench or canteen all through
Recently, Semba reviewed early literature on fish liver       the year … If close contact were all, one would think the
oils given as an ‘anti-infective’ [97]. These oils contain   London Transport would ensure an all-the-year epidemic.
                                                                     Epidemic influenza and vitamin D          1135

Summertime deaths due to influenza are rare except             The critical question of ‘ What is an ideal 25(OH)D
during pandemics – even during pandemics, most             level ?’ must be answered, ‘In regard to what ? ’ Levels
deaths occur during the colder months. Nor does the        needed to prevent rickets and osteomalacia (10 ng/ml)
indoor theory of contagion explain why the adminis-        are lower than those that dramatically suppress
tration of live attenuated influenza virus produces         parathormone levels (20 ng/ml) [105]. In turn, those
such seasonal results in non-immune volunteers, while      levels are lower than those needed to increase intes-
the vitamin D theory would predict exactly such            tinal calcium absorption maximally (34 ng/ml) [106].
observations due to the stimulation of antimicrobial       In turn, neuromuscular performance in 4100 elderly
proteins and a suppression of cytokine response            patients steadily improved as 25(OH)D levels in-
during the summer when UVB radiation induces the           creased and maximum performance was associated
production of vitamin D. An indoor theory of con-          with levels of 50 ng/ml [107]. If levels of 50 ng/ml are
tagion has difficulty explaining the strong association      associated with further benefits, such as preventing
between vitamin D-deficient rickets (and simple             viral respiratory infections, we are only now learning
vitamin D deficiency in non-rachitic children) and          about it. Until more is known, it may be prudent
childhood infections, while the vitamin D theory           to maintain wintertime 25(OH)D at concentrations
explains both. Furthermore, the indoor contagion           achieved in nature by summertime sun exposure
theory cannot explain why age-adjusted influenza            (50 ng/ml).
deaths are more common among African-Americans                There are a number of factors to consider regarding
than whites, whereas the striking racial differences in     the most appropriate dose of vitamin D. One minimal
25(OH)D levels readily explain it. Nor can an indoor       erythemal exposure of the full-body to artificial UVB
theory explain the observations regarding sunlight,        radiation triggers the release of about 20 000 IU of
artificial UVB, cod liver oil, and supplemental             vitamin D into the circulation of light-skinned
vitamin D.                                                 persons within 48 h [108]. There was no evidence of
   Eccles proposed that cooling of the nasal airways,      toxicity in young men taking 50 000 IU of vitamin D
which reduces mucociliary clearance and phagocytic         a day for 6 weeks (although such a dose would be
activity, may explain the seasonality of viral respirat-   toxic if taken over a longer period) [109]. In 32
ory infections [103]. The same group produced evi-         vitamin D-deficient elderly patients, 50 000 IU/day of
dence in a controlled trial that chilling of the feet      vitamin D for 10 days showed no evidence of toxicity
causes about 10% more subjects than controls to            and only raised 25(OH)D levels by an average of
report the delayed onset of cold symptoms [104].           5 ng/ml 3 months after administration and in no
Furthermore, the common folklore that respiratory          patient did levels exceed 13 ng/ml at 3 months
infections often follow exposure to cold air or to         [110]. Single injections of 600 000 IU (15 mg) raised
chilling of the body by wet hair, feet, or clothes is      25(OH)D levels from 2 ng/ml to 22 ng/ml at 2 weeks
pervasive and unlikely to be entirely superstitious.       and to 26 ng/ml at 6 weeks in ten elderly subjects with
Although this theory has evidence to support it and        no evidence of toxicity [111]. Indeed, a single injection
may explain some of the seasonality of respiratory         of 600 000 IU of vitamin D is safe ; such doses were
infections, it, like the indoor contagion theory, fails    recently recommended for the elderly to prevent
to explain the observations detailed above while the       vitamin D deficiency [112]. These studies indicate
vitamin D theory has plausible, albeit inadequately        short-term administration of pharmacological doses
tested, explanations for them all.                         of vitamin D is safe.
                                                              A vitamin D intake of 2000 IU/day for 1 year failed
                                                           to achieve a 32 ng/ml target 25(OH)D concentration
Dosage of vitamin D
                                                           in 40% of the post-menopausal African-American
The seasonality hypothesis proposed here relates to        women studied [113]. Administration of 4000 IU/day
sun-derived vitamin D. However, if vitamin D might         of vitamin D for more than 6 months to middle-age
be effective in preventing seasonal respiratory infec-      Canadian endocrinology outpatients, resulted in
tions, then the daily oral dosage required for an effect    average 25(OH)D levels of 44 ng/ml and produced
remains to be addressed. Both the hypothesis and the       no side-effects other than an improved mood
dosage must be addressed through properly conducted        [114]. Heaney has estimated that about 3000 IU/day
clinical trials. A likely dose requirement can be esti-    of vitamin D is required to assure that 97% of
mated from existing knowledge of vitamin D nutrition.      Americans obtain levels >35 ng/ml [115]. Dosage will
1136     J. J. Cannell and others

depend upon age, latitude, season, skin type, body         D less likely to become infected ? Should the concept
weight, sun exposure, and pre-existing 25(OH)D             of human herd immunity (the immune pressure on the
levels. Some groups – African-Americans, the obese,        virus due to the percentage of the population with
and the elderly – may require supplementation with         acquired immunity) be expanded to include innate
5000 IU/day during winter but less, or none, during        herd immunity [the immune pressure on the virus
the summer to obtain 25(OH)D levels of 50 ng/ml.           due to the percentage of the population with adequate
These studies indicate that ideal daily doses of vitamin   25(OH)D levels] ? Is influenza infection a sign of
D exceed current recommendations by an order of            vitamin D deficiency as much as Pneumocystis carinii
magnitude.                                                 pneumonia is a sign of AIDS ? Does the adminis-
   If the ability of vitamin D to stimulate the pro-       tration of pharmacological doses of vitamin D,
duction of virucidal antimicrobial peptides and to         early in the course of a viral respiratory infection,
suppress cytokine and chemokine production is              ameliorate symptoms ? As the annual mortality from
clinically significant, then pharmacological doses          influenza approaches one million worldwide, further
(1000–2000 IU/kg per day for several days) may be          studies testing this theory are warranted [119].
useful in the treatment of those viral respiratory            Today, in a rush from multiplex reverse trans-
infections that peak in wintertime. Physicians have        criptase–polymerase chain reactions that rapidly
successfully used pharmacological doses of vitamin D       subtype influenza viruses to complex mathematical
to prevent vitamin D deficiency, to prevent metabolic       formulas that explain its infectivity, many of us have
bone disease, and to treat severe hypoparathyroidism.      forgotten Hope-Simpson’s simple ‘seasonal stimulus ’
Perhaps such doses have other effects, such as ameli-       theory for the lethal crop of influenza that sprouts
orating symptoms of viral respiratory infections. As       around the winter solstice. The faith and humility that
pointed out in a 1999 paper that heralded the current      characterized his life and his writings insulated him
interest in the nutrient, the pharmacological potential    from despairing that his ‘seasonal stimulus ’ would
of vitamin D remains unexplored [116].                     not be sought. Among his last published words was
                                                           the suggestion that ‘it might be rewarding if persons,
                                                           who are in a position to do so, will look more closely
C ON C L U S IO N
                                                           at the operative mechanisms that are causing such
There is much evidence to suggest that vitamin D may       seasonal behavior ’ [3, p. 241].
be Hope-Simpson’s seasonal stimulus. Nevertheless,
it is premature to recommend vitamin D for either
the prevention or treatment of viral respiratory           ACKNOWLEDGEMENTS
infections. It is not, however, too early to recommend
                                                           We thank Professor Norman Noah of the London
that health-care providers aggressively diagnose
                                                           School of Hygiene and Tropical Medicine, Professor
and adequately treat vitamin D deficiency. Vitamin D
                                                           Robert Scragg of the University of Auckland and
deficiency is endemic and has been associated with
                                                           Professor Robert Heaney of Creighton University for
many of the diseases of civilization [117, 118]. Vitamin
                                                           reviewing the manuscript and making many useful
D supplementation should stabilize 25(OH)D con-
                                                           suggestions.
centrations consistent with levels obtained by natural
summertime sun exposure (50 ng/ml) while avoiding
toxic levels. Those with large amounts of melanin in
their skin, the obese, those who avoid the sun, and        D E C L A R A T I O N O F IN T E R E S T
the aged may need up to 5000 IU/day to obtain such         Dr Cannell heads the non-profit educational group,
levels, especially in the winter.                          ‘ The Vitamin D Council ’.
   The theory that vitamin D affects the course of viral
respiratory infections should be tested. Are patients
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