How are the viruses for flu vaccine selected?

Each year, many labs 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 labs, 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 different subtypes of influenza A viruses and one influenza B virus) to go into flu shots for the following fall and winter. Usually one or two of the three virus strains change in the vaccine each year.

Why Do I Need to Get a Flu Shot 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 hold up when a new strain is circulating. Second, a vaccine made against one flu virus may not protect against the newer viruses. That is why the flu vaccine is updated to include the current viruses every year.

A third reason why you should get a flu shot every year is that after you get a flu shot, your immunity to the flu declines over time and may be too low to provide protection after 1 year.

Does a Flu Shot Work Right Away?

No. It takes about 2 weeks after the shot for antibodies to develop in the body and provide protection against influenza virus infection. In the meantime, you are still at risk of getting the flu. That’s why it’s better to get your flu shot early in the fall, before the flu season really gets under way. Historically, the flu season in Minnesota and Wisconsin starts around Christmas, but it can be as late as March or April.

Protection from influenza lasts up to 6 months after receiving a vaccination.

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