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.
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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.
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Why
should people get vaccinated against the flu?
Influenza
is a serious disease, and people of any age can get it. In
an average year, the flu causes 36,000 deaths (mostly among
those aged 65 years or older) and more
than 200,000 hospitalizations in the United States. The
"flu season" in the United States is usually from
November through April each year. During this time, flu viruses
are circulating in the population. An annual flu vaccine (either
the flu shot or the nasal-spray flu vaccine) is the best way
to reduce the chances that you will get the flu.
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When
should I get a flu vaccination?
Beginning
each September, the flu shot should be offered to people at
high risk when they are seen by health-care providers for
routine care or as a result of hospitalization. See the ACIP
Recommendations for Using Inactivated Influenza Vaccine.
The
best time to get vaccinated is from October through November.
Flu activity in the United States generally peaks between
late December and early March.
You
can still benefit from getting vaccinated after November,
even if flu is present in your community. Vaccine should continue
to be offered to unvaccinated people throughout the flu season
as long as vaccine is still available. Once you get vaccinated,
your body makes protective antibodies in about two weeks.
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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.
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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.
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How
effective is the flu vaccine?
The
flu vaccine is the most effective way to prevent the flu.
However, in limited studies, the flu shot and the nasal-spray
flu vaccine (LAIV) have different rates of effectiveness.
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If
I got a vaccine last year, will that protect against the flu
for this year?
Vaccination
last year is unlikely to protect against influenza this year
because a person's immunity after influenza vaccination declines
over the year after vaccination. This is one reason why it
is recommended that people get vaccinated every year.
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Why
do I need to get vaccinated against the flu every year?
Flu
viruses change from year to year, which means two things.
First, you can get the flu more than once during your lifetime.
The immunity (natural protection that develops against a disease
after a person has had that disease) that is built up from
having the flu caused by one virus strain doesn't always provide
protection when a new strain is circulating. Second, a vaccine
made against flu viruses circulating last year may not protect
against the newer viruses. That is why the influenza vaccine
is updated to include current viruses every year.
Another
reason to get flu vaccine every year is that after you get
vaccinated, your immunity to the disease declines over time
and may be too low to provide protection after one year.
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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.
