Dear Doctor: If I received the flu shot in September, will I still be protected from the flu in March?
Dear Reader: That's a very good question -- and the answer depends, in part, on your current health.
The flu season normally lasts from November to March, but it can begin in October and even end in May. The injected influenza vaccine, which contains three to four inactivated viruses from the prior flu season, spurs the formation of antibodies to help your body fight off influenza if you're exposed. It takes about two weeks after the vaccine for your body to produce sufficient protective antibodies.
If you are a young, healthy person under 50, you're more likely to get a good immune response from the vaccine -- and also more likely to have an immune response that lasts six to 12 months. You'll also have a greater and longer-lasting immune response if you get the flu vaccine every year. So if you got your shot in September, you should have immunity that lasts throughout the flu season.
If you're older than 50 and have diabetes, cancer, or kidney or liver failure, your ability to mount a good response from the vaccine decreases. So six months after your vaccination, you may not have sufficient antibodies to ward off the flu. This has been shown specifically for the H1N1 influenza virus. If you received the vaccine on Sept. 1, you may be less protected by April of the following year against H1N1.
This decreased immunity is especially pronounced in people over the age of 65. It decreases even further among those over 65 who have chronic illnesses.
Some pharmacies have been administering the flu vaccine as early as the middle of August. This isn't necessarily a bad thing for someone older than 65 or someone with chronic illnesses, because you still get immunity from the vaccine that should last through much of the flu season. Still, it's probably better to wait until at least the middle of September to get the flu vaccine just to ensure you're protected for as much of the flu season as possible.
Finally, it's especially important to get yearly flu vaccines if you are: older than 60; between the ages of 6 months and 4 years; have chronic illnesses; or are pregnant. The flu is especially risky for these groups.
While imperfect, the flu vaccine is the best way our society has found to decrease the death rates and the long-term complications from a potentially fatal disease that causes anywhere from 3,300 to 49,000 deaths per year in the United States.
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