Questions & Answers: Flu Vaccine from
Updated: Dec 10, 2003
What kind of flu vaccines are there?
There are two types of vaccines that protect against the flu. The "flu shot" is an inactivated vaccine (containing killed
virus) that is given with a needle, usually in the arm. A different kind of vaccine, called the nasal-spray flu vaccine
(sometimes referred to as LAIV for Live Attenuated Influenza Vaccine), was approved in 2003. The nasal-spray flu
vaccine contains attenuated (weakened) live viruses, and is administered by nasal sprayer. It is approved for use
only among healthy people between the ages of 5 and 49 years. The flu shot is approved for use among people
over 6 months of age, including healthy people and those with chronic medical conditions.
Each of the two vaccines contains three influenza viruses, representing one of the three groups of viruses
circulating among people in a given year. Each of the three vaccine strains in both vaccines – one A (H3N2) virus,
one A (H1N1) virus, and one B virus – are representative of the influenza vaccine strains recommended for that
year. Viruses for both vaccines are grown in eggs.
Updated Dec 10, 2003
How do flu vaccines work?
Both flu vaccines (the flu shot and the nasal-spray flu vaccine (LAIV)) work in the same way; they cause antibodies
to develop in the body, and these antibodies provide protection against influenza virus infection..
Updated Dec 10, 2003
Does flu vaccine work right away?
No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against
influenza virus infection. In the meantime, you are still at risk for getting the flu. That's why it's better to get
vaccinated early in the fall, before the flu season really gets under way.
Updated Dec 10, 2003
Can I get the flu even though I got a flu vaccine this year?
Yes. The ability of flu vaccine to protect a person depends on two things: 1) the age and health status of the person
getting the vaccine, and 2) the similarity or "match" between the virus strains in the vaccine and those in circulation.
ted Dec 10, 2003
do I need to get vaccinated against the flu every year?
ruses change from year to year, which means two things. First, you can get the flu more than once during your
me. The immunity (natural protection that develops against a disease after a person has had that disease) that is built
om having the flu caused by one virus strain doesn't always provide protection when a new strain is circulating.
nd, a vaccine made against flu viruses circulating last year may not protect against the newer viruses. That is why the
nza vaccine is updated to include current viruses every year. Another reason to get flu vaccine every year is that
you get vaccinated, your immunity to the disease declines over time and may be too low to provide protection after
Updated Dec 10, 2003
How are the viruses for flu vaccine selected?
Each year, many laboratories throughout the world, including in the United States, collect flu viruses. Some of these
flu viruses are sent to one of four World Health Organization (WHO) reference laboratories, one of which is at the
Centers for Disease Control and Prevention (CDC) in Atlanta, for detailed testing. These laboratories also test how
well antibodies made to the current vaccine react to the circulating virus and new flu viruses. This information, along
with information about flu activity, is summarized and presented to an advisory committee of the U.S. Food and
Drug Administration (FDA) and at a WHO meeting. These meetings result in the selection of three viruses (two
subtypes of influenza A viruses and one influenza B virus) to go into flu vaccines for the following fall and winter.
Usually, one or two of the three virus strains in the vaccine are changed each year.
Part 4: How are Flu Vaccines Made?
Each year in January, production lines gear up to produce between 12 and 30 million doses of flu
vaccine — each designed to protect against three different flu strains. The tough part is deciding which
three strains to include. Not only must vaccine makers judge which strains are different enough to
trouble the immune system, they must also determine which strains spread most easily. In making
those decisions, flu experts make scientifically-based predictions that are literally matters of life and
On what data are those predictions based? The Center for Disease Control in Atlanta constantly
receives virus samples from all over the world. Each sample is logged into a database that records
where the strain came from, whether it caused serious illness, and whether it infected only a few
isolated people or seemed to be spreading rapidly. Each strain is genetically fingerprinted and
compared with known strains. Parts of the original samples are stored in a deep freeze, and parts are
inoculated into chicken eggs or special cell cultures to be grown for further study.
Samples whose coat proteins look "new" enough to cause trouble are tested to see how infectious they
are, and how long it takes the immune system to respond to them. Throughout the entire process, the
cultured strains are monitored to make certain that they haven't mutated too much from the original
samples. In the meantime, researchers in the lab stay in constant contact with public health officials and
fieldworkers in the parts of the world where the most "interesting" strains originated. Why? Because, for
reasons that still aren¹t understood, some strains spread quickly in human populations, while others just
don't get around very much.
The results of all these studies are combined, analyzed, and presented to expert panels at a meeting of
the U.S. Food and Drug Administration (FDA) Vaccines and Related Biological Products Advisory
Committee. Then the debates begin. Which strains produce the most serious symptoms? Which
spreads from person to person most effectively? Which strains seem to be emerging from the areas in
which they first appeared? Which are staying put? Which of the potentially serious strains can be grown
well enough in the lab to produce enough vaccine?
Finally, the experts use these data to make the best predictions they can about which three strains are
most likely to cause the most trouble in the United States the following winter.
Once the decision is made, the rush to manufacture vaccine is on. For the present, vaccines are still
grown in an "old fashioned" way. Chosen viral strains are inoculated into 90 million fertilized chicken
eggs in which the viruses are incubated. The eggs are opened, and the virus is extracted and purified.
Then the virus is treated chemically to kill it while preserving the normal shape of its coat proteins. Each
final dose of vaccine is very small, containing scarcely fifteen millionths of a gram of each of three
different viral strains. Yet that tiny amount of foreign protein, injected into your body, is enough to place
your entire immune system on red alert. Assuming that you are vaccinated at least a week before you
are exposed to live virus, your B-cells and T-cells will be ready to defend you — but only against the
strains contained in the vaccine.