You might have gotten a flu shot last year. Nearly half of American adults did. But guess what: You have to start all over again this year. Last year’s vaccine is about as timely as last week’s meme.
The first reason is that there isn’t just one flu virus, explains Michelle Barron, MD. She’s the senior medical director for infection prevention and control at UCHealth in Denver. “It’s actually a family of viruses,” she says.
The second reason is that the flu viruses themselves change every year, says Adolfo Garcia-Sastre, PhD. He’s the director of the Global Health and Emerging Pathogens Institute at Icahn School of Medicine at Mount Sinai in New York City. That makes it easier for new strains to bypass your immune system.
That variability makes the flu virus different from measles or chicken pox — and it’s why the flu shot isn’t a one-and-done thing, says Dr. Barron. Here is a rundown of the factors that go into making a yearly flu vaccine and how to find the one that will protect you best.
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The flu virus can change, a little or a lot
Scientists call these changes “drift and shift.” Drift is more common. It’s what happens when the virus makes copies of itself. “Imagine if, instead of having a copy machine, you had to write the same sentence 1,000 times,” Dr. Barron explains. “At some point, you’re going to make an error. And that mistake is repeated because you’re tired.”
Usually, those tiny changes are minor. But over time, they add up. So older versions of the flu vaccine won’t provide good protection anymore.
Shift is when a virus jumps to humans from animals, such as from pigs or birds. That’s more dramatic. In that case, the virus has never been seen in humans before. That’s what happened during the 2009 swine flu pandemic. So scientists had to scramble to develop a vaccine for it.
What happens around the globe is important
Every 6 months, new flu strains affect half of the globe. When it’s summer in the U.S., it’s winter in countries in South America, parts of Africa and Australia. That means it’s flu season for those areas of the world.
The reverse is true, too. When it’s summer in the southern hemisphere, it’s winter here, in Europe and in other northern countries. That’s when flu season hits hardest in the U.S.
That’s important information for the scientists who are making this year’s vaccine. U.S. scientists pay attention to the strains of flu that circulated the most in the southern hemisphere earlier in the year. Then they make a vaccine to protect against those strains.
Scientists in the southern hemisphere do the same thing. They observe the flu virus strains that affected people in the north. Then they come up with a vaccine for the next flu season based on those circulating strains.
The flu vaccine may not be exact
All of the flu vaccines this year are known as “quadrivalent.” They protect against 2 influenza virus A strains and 2 influenza B strains. A and B strains affect humans the most.
But picking the right strains is like betting on a horse race, explains Dr. Barron. “Most of the time, the winner is going to be what you predicted based on the odds,” she says. “But every now and then, out of left field, your 100-to-1 chance comes through.”
But a partial match is better than none at all, she notes. Even if you come down with the flu after your shot, your likelihood of getting very sick goes down. That’s especially true of people over 65 years old and little kids. And that’s good news, since they’re the ones who are most likely to go to the hospital with complications from the flu.
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