Family Medicine

Why do some people still get the flu even after they get a flu shot?

Although flu season usually begins in mid- October, researchers at the U.S. Centers for Disease Control and Prevention (CDC) start months earlier, trying to identify the strains of influenza around the world that are most likely to affect people in this country. The resulting vaccine is designed to protect against the three strains expected to cause the most illnesses during flu season.

Most years, the vaccine matches the predominant strain of flu that arrives in the United States. In fact, CDC records indicate that the vaccine has matched the predominant strain of flu virus in 15 of the past 16 years. When the vaccine is a good match to the predominant strain, the vaccine will prevent flu in 70 to 90 percent of healthy adults. And when individuals who have been vaccinated come in contact with a strain of the flu not covered by the vaccine, they can become ill, but their vaccination should still offer some protection against that new strain, particularly if it is closely related to the ones covered by the vaccine.

You can also get the flu if you come in contact with the virus too soon after being vaccinated. After getting a flu vaccine, your body’s immune system needs about 10 days to manufacture the antibodies it needs to fight off the flu, so it is possible to come down with the flu before your immune system is fully armed against the virus. This situation has probably led some individuals to think that the vaccine caused their bout with the flu.

Like all living things, flu viruses are continually changing. Normally, these changes usually occur gradually over time. On occasion, however, a flu virus will undergo what’s known as an “antigenic shift.” When this happens the changes to the virus are so abrupt and so significant that few people will be protected against it.

Even though you can still come down with the flu after you’ve been vaccinated, you should make it an annual tradition to get a flu vaccine. You’ll be much less likely to get the flu or to pass the illness to another individual. Flu season in the U.S. usually peaks in mid-winter, but you should plan to get vaccinated early so that you are protected before the beginning of the holiday season. In November and December, people tend to gather (think holiday parties or shopping malls) providing countless opportunities for virus transmission.

This year, a record number of vaccine doses will be available and the CDC has changed its recommendations about who should be vaccinated. That group generally covers anyone who wants to be vaccinated, but should include: children between the ages of six months and 19 years; pregnant women; people older than 50; people with chronic medical conditions; people who live in nursing homes or long-term care facilities; healthcare workers and people who live with individuals who are at high risk of complications from the flu.

Originally published in The University Doctors' MedicaLink - 10/08

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