A vCJD Risk-Assessment Case Study
A Preliminary Quantitative Assessment of Canadian Vaccines
Incorporating Bovine Biological Materials in Their Manufacture
Susie ElSaadany and Antonio Giulivi
Reprinted with permission from BioProcess International 3(3): pp 62-68 (March 2005)
In 1986, bovine spongiform Kingdom, the manufacturer of an oral vaccine manufacture (Tables 2 and 3).
encephalopathy (BSE), a new polio vaccine recalled and removed it One general assumption is that the
transmissible spongiform from the market after discovering that infectivity of bovine-derived material
encephalopathy (TSE), was the virus stocks or master/working cell is equal to the infectivity of ovine-
recognized in the United Kingdom banks used for the vaccine’s derived material (7). Because BSE is a
(UK) (1). In 1996, the Creutzfeldt- manufacture had incorporated fetal neural disease with its greatest
Jakob Disease (CJD) Surveillance calf serum (FCS) sourced locally (5). infectivity found in neural tissue (7),
Unit in Edinburgh identified a new The following preliminary bovine brain is assumed to be 100
variant of the disease (vCJD) in quantitative risk-assessment modeling million times more infective than
humans that exhibited characteristics exercise was undertaken to help bovine muscle. If bovine brain
linking it to BSE-infected animals (1). inform decision makers about the contains 10 million infective doses (8,
Discussion in the UK BSE Inquiry potential risk to the Canadian public 9), then it can be assumed that bovine
allowed for consideration that vaccines of vCJD from vaccines administered muscle is 100 million times less
might transmit the BSE agent to from 1975 to 2001. infective and contains 0.1 infective
humans (1, 2). If vaccines routinely doses per gram. Another assumption
administered in childhood could Methods is that all people are susceptible to
transmit BSE, and if vCJD was first A non-vaccine-specific overview of vCJD (with no specific genetic
detected in 1996, and with the average animal-derived components in the susceptibility of patients with
age at onset of vCJD at 28 years of vaccine manufacturing process was methionine-methionine variation at
age (3), then BSE might have conducted to model worst-case codon 129 in the PrP gene), and no
originated sometime in the early 1970s historical and current risk to reduction factor has been assigned. It
and not in the early 1980s, as was Canadians from vCJD through the is assumed that there is no species
originally thought. use of bacterial and viral vaccines barrier for BSE from bovines to
As more became known about (Table 1). The exercise assumes, humans. All data used in the
vCJD in the early 2000s, and as BSE though it is unlikely, that the calculations are assumed to be
was detected in other parts of Europe manufacture of those vaccines applicable during the peak of the BSE
(from which, subsequently, bovine included BBM sourced from BSE- epidemic in the United Kingdom and
biological material could no longer be reporting countries. We considered Europe (1980–1992). We also assumed
sourced due to regulatory vaccines incorporating BBM (beef that the risk outputs (concentrations)
requirements), efforts increased to broth and fetal calf serum) that were are equal for the master seed and
qualify and quantify the risk of BSE potentially manufactured before working seed in viral vaccine
from vaccines. When the BSE Inquiry institution of safety measures banning manufacture, and equal for the master
proposed the presence of BSE in the the sourcing of BBM from countries seed, working seed, and fermentor for
1970s (4), it acknowledged the with BSE, and which might still have bacterial vaccine manufacture. Table 2
possibility that master/working cell been included in existing stocks and shows the estimated infectivity of
banks or seed stocks for vaccines administered between 1975 and 2001. bovine tissue by category (10).
developed at that time using bovine Our risk-assessment equations Calculating the risk from viral
biological material (BBM) may have incorporate many assumptions vaccines manufactured with fetal calf
been contaminated. In the United regarding raw materials used in serum requires the following
62 BioProcess International M arch 2005
assumptions. Sera from approximately
1500 calves are pooled for the
manufacture of fetal calf serum (7).
The assessment, therefore, created
scenarios to accommodate a range of
1000–2000 calves. The incidence of
BSE in adult cattle is assumed to have
been about 1/200 during the mid-
1980s, the time at which the FCS was
used to establish master and working
cell lots (7). Maternal-fetal
transmission rate may have ranged
from 10% (11) up to as high as 20%.
Scenarios were created to
accommodate a range from less than
10% to a maximum of 20%. We
assumed that the serum of an infected
calf could contain up to one infectious
unit per mL (7–12). The number of
infectious BSE units that potentially
entered the vaccine production process
would be equal to the number of
infectious units that remain in the
vaccine at the end. (The vCJD risk is
the input number of infectious units
divided by the number of doses of
vaccine in the batch). We assumed an
absence of effective purification steps pooling of animal material for beef and the effect of that processing on
to eliminate infectivity during vaccine broth and that material from only one the survival and inactivation or
processing and that the BSE agent animal was used (7). An additional physical presence of BSE in the final
does not replicate during the scenario was created where material product. Vaccines of these types have
manufacturing process. Our from five to 10 cattle was pooled in been administered to all Canadian
assessment considered that the entire the broth used for the manufacture of children who received routine
birth cohort of Canada, some 400,000 bacterial vaccine. The nutrient vaccinations from infancy to 16 years
children for any given year, was medium used to grow bacteria for the of age from between 1975 and 2001.
vaccinated with those products. In the vaccine contained approximately 750 Statistical analysis was conducted
risk estimate, a factor of 200 is grams of skeletal muscle and 200 using decision-making techniques that
allowed for reduced transmission by grams of pancreatic extract (7). This used scenario analysis (estimating
the intramuscular (parenteral) route. It creates two scenarios: one assuming possible outcomes) followed by Monte
is assumed that every child receives that the media component of skeletal Carlo simulation (100,000 runs).
eight doese of viral vaccine and that muscle (Category IV) is free of Monte Carlo simulation was used to
each dose carries the same risk (12). In nervous tissue contamination and the predict critical outcomes based on the
the absence of information at the time other assuming that Category IV impact and sensitivity of theoretical
of this assessment, we assumed that components are 0.01% contaminated factors. The analysis used available
inactivated polio vaccine also may with Category 1 material (neural information to estimate the theoretical
have been manufactured with risky tissue) (13). The incidence of BSE in risks. Tables 4 and 5 show input
material during the time period European cattle is one in 10,000. The variables for viral and bacterial vaccine
studied. average BSE rate in Europe is risk equations.
For calculating risk from bacterial multiplied by a factor of 10 to account
vaccines manufactured with beef for uncertainty (10). A reduction Results
broth, we assumed that the broth was factor of 20 is assigned to the The risk of vCJD to the Canadian
not sourced from the United autoclaving (wet high temperature population for viral vaccines
Kingdom, but from other European treatment) of the broth (7). It is manufactured with fetal calf serum
countries. Table 3 details the Risk assumed that every child receives 11 was estimated as follows (Table 6):
Categories for Bovine Media doses of bacterial vaccine and that
Components Used in Manufacture of each carried the same risk (12). T
Bacterial Vaccine (2,13). Our Information gaps include the treatment for each child is
calculations assumed that there was no method of raw materials processing estimated at 8 × 10−9, with a
M arch 2005 BioProcess International 63
lower limit of certainty of 1 × manufactured with beef broth was children, with a lower limit of
10−9 and an upper limit of estimated to be as follows (Table 7): certainty of 0.001 and an upper
certainty of 1 × 10−7 limit of certainty of 0.663.
• heestimatedtotalnumberof T
children infected in the birth treatment per child is estimated at
cohort is estimated at 0.0003 6 × 10−9, with a lower limit of discussion
children, with a lower limit of certainty of 5 × 10−8 and an Every risk factor and model parameter
certainty of 0.00009 and an upper upper limit of certainty of 1 × carries its own uncertainty, expressed
limit of certainty of 0.006 10−5. through the confidence limits
• heestimatedtotalnumberof surrounding the estimates. Extracts or
The risk of vCJD to the Canadian children infected in the birth infusions of bovine tissues are
population for bacterial vaccines cohort is estimated at 0.002 considered to carry the highest risk for
64 BioProcess International M arch 2005
fermentation broths and are used for which bovine material was sourced. accommodated into our model as the
this worst-case scenario analysis. All materials of concern (e.g., information becomes available.
Monte Carlo simulation was the mixtures) may not be known. No Purification Processes: A
method of choice to accommodate the distinction or accommodation has worthwhile goal for future refinement
uncertainty caused by lack of been made for cattle reaching of risk estimates may be to study
information. The model may become sufficient age to exhibit BSE whether purification processes during
more robust, and a more sophisticated symptoms. The incubation time of vaccine production eliminate BSE.
approach may be used to vCJD in humans is not taken into The current model makes no
accommodate new or additional data. account. For the master seed/reference assumptions in that regard.
The years during which the vaccines and working seed, the amount and Conversely, an assumption is made
were distributed to the Canadian type of bovine material used for that the vaccine production process
population have not been concretely dilution throughout the does not replicate prions that may
identified, nor are the years during manufacturing process will be have been present in seed stocks or
M arch 2005 BioProcess International 65
12 National Advisory Committee on
BBM, nor result in their concentration manuscript drafting and revision of Immunization. Recommended Childhood
Vaccination Schedule. Division of
in the final product. The question content, the authors acknowledge Ms.
Immunization, Health Canada; External Web
then is whether changes in BSE Natalie Hanna, communication site .
“spiked” material should be officer, and Dr. Mohammad Afzal, 13 Vann W. Bacterial Vaccines: Overview
experimentally quantified through the senior biostatistician of the Statistics of Manufacturing and Risk Assessment.
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been done for prions in human plasma Safety Surveillance and Health Care Transmissible Spongiform Encephalopathies
Advisory Committee and Vaccines Related
fractionation. That would validate Acquired Infections Division, Centre
Biological Products Advisory Committee. 27
certain “dilution” steps that have been for Infectious Disease Prevention and July 2000; External Web site .
assumed to occur. Additionally, the Control, Public Health Agency of
assessment has not considered the Canada.
possibility of risk material being in Corresponding author Susie ElSaadany is
fillers and stabilizers of the final RefeRences: chief of the Statistics and Risk Assessment
vaccine product. It would be valuable 1 Government of the United Kingdom, Section, Blood Safety Surveillance and
to have detailed information regarding House of Commons. The BSE Inquiry, Health Care Acquired Infections Division,
Volume 1: Findings and Conclusions. Centre for Infectious Disease Prevention
changes over time in vaccine Executive Summary of the Report of the and Control, Public Health Agency of
manufacture that would reduce the Inquiry, Section 8 of the executive summary, Canada, A.L. 0601E2/Health Canada
risk vCJD contamination. Medicines. London: The Stationery Office, Building No.6, Tunney’s Pasture, Ottawa,
As a worst-case analysis, this 2000: xxix–xxvi; External Web site .
Ontario K1A 0L2; 1-613-952-6925; fax: 1-613-
assessment may overestimate 2 Government of the United Kingdom - 952-6668; email@example.com.
Canadian risk, assuming as it does House of Commons. The BSE Inquiry.
Antonio Giulivi (MD, FRCP(C)) is director of
Volume 7 of the inquiry report, Medicines and
that all beef broth was sourced from Cosmetics. London: The Stationery Office,
the Blood Safety Surveillance and Health
Europe. The risk assessment addresses 2000: 1, 37–39, 56–91, 100–101, 126–130, Care Acquired Infections Division, Centre for
only those vaccines recommended for 223. Infectious Disease Prevention and Control,
routine administration by Health 3 US Food and Drug Administration. Public Health Agency of Canada.
Canada’s National Advisory Questions and Answers on Variant CJD. 2001,
Committee on Immunization External Web site
(NACI). It does not necessarily 4 Government of the United Kingdom -
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include other vaccines that may be Volume 1:. Findings and Conclusions.
routinely administered to children, Executive Summary of the Report of the
such as varicella vaccine and flu shots, Inquiry Volume 8, Medicines. London: The
as well as those approved for use in Stationery Office, 2000. xvii-xxxi, 14–19, 121,
166, 177, 192, 220, 261.
children but not part of a routine
5 Department of Health, United Kingdom.
immunization schedule. Additional
Recall of Medeva Oral Polio Vaccine. 20
data on the quantity of risky material October 2000, External Web site .
used throughout the manufacturing 6 National Advisory Committee on
process for the vaccines in question is Immunization, Health Canada. Recommended
required for a more sophisticated Immunization for Infants, Children and
assessment. Determining such data Adults. Canadian Immunization Guide, Fifth
Edition, Part 2: 45–47, 1998; External Web
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11 Wilesmith JW, et al. A Cohort Study
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For contributions to acquisition of Encephalopathy. The Vet Record 141, 1997:
data and editorial work including 23–243.
66 BioProcess International M arch 2005