Q. Does a flu shot work right away?
No. It takes about two weeks after the 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.
Q. Can I get the flu even though I got a flu shot this year?
Yes. The ability of a flu shot to protect a person depends on two things: 1) the age and health status of the person getting the shot, and 2) the similarity or "match" between the virus strains in the vaccine and those in circulation. When the "match" between vaccine and circulating strains is close, flu vaccine prevents influenza in about 70%-90% of healthy persons younger than age 65 years.
Among elderly persons living outside chronic-care facilities (such as nursing homes) and those persons with long-term (chronic) medical conditions, flu vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza.
Among elderly nursing home residents, flu vaccine is most effective in preventing severe illness, secondary complications, and deaths related to the flu. In this population, the vaccine can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from the flu.
Many people think that "the flu" is any illness with fever or cold-like symptoms, or any illness with stomach or intestinal problems, such as vomiting, nausea, or diarrhea. They may expect flu vaccine to prevent these illnesseses. But flu shots are effective only against illness caused by flu viruses, and not against other causes of fever, colds, or gastro-intestinal illnesses caused by other viruses.
Q. How are the viruses for flu shots selected?
Flu Strains for Vaccine Are Carefully 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 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.