Is post-flu immunity better than vaccine?

Does surviving the flu provide better immunity than a flu shot?

Getting the flu itself can provide much stronger immunity than any flu shot does -- under some circumstances. But getting the flu is dangerous.

When a flu virus enters the body, the immune system fights the infection by producing a robust antibody response that can spring into action if the same strain of the virus returns. That protection can, in some cases, last a lifetime. "The evidence we have is that people who were infected back in the '30s still have immunity to those viruses," said Dr. Alicia Fry, a medical epidemiologist at the Centers for Disease Control and Prevention.

Even when the antibody response wanes, immunity from an infection tends to last longer than that from a vaccine. A 2011 study in PLoS One, for example, found that just over half of patients infected with the 2009 H1N1 flu still had an antibody response after six months, while only about a third of vaccinated subjects did.

So why isn't natural immunity better? For one, natural infection makes people sick. "Some people end up in the hospital, and some people die," Fry said.

The risks are particularly concerning for older people, those with chronic diseases, pregnant women and young children. A healthy person who survives a bout of illness without harm still exposes other people, who might not be not so fortunate, to the virus.

Further, natural immunity to one flu virus won't protect a body from most other strains of the virus.

"If you were infected with a specific flu virus, your natural immunity would be very good at protecting you against that virus or a similar virus in the future," Fry said. But flu viruses evolve quickly and can change from year to year, and several strains of virus typically circulate in any given year.

The flu vaccine is updated annually to target the strains most likely to circulate. That, along with its fleeting protection, is why health authorities recommend getting a new flu shot every year.

Finally, there's no guarantee you'll get a robust immune response from infection. It depends on several factors, including your previous exposure to flu viruses, your body's ability to respond and how much of the virus you're exposed to, according to Dr. Suryaprakash Sambhara, an immunologist at the CDC.

For perspective, a 2008 study in Nature found that survivors of the 1918 Spanish flu earned lifelong immunity to that flu virus and, as it turned out, protection against the pandemic H1N1 flu of 2009. But the 1918 flu killed 50 million people worldwide.

Dr. Eric L. Altschuler, a professor at Temple University and an author of that study, said it would be unwise to expose yourself to one flu virus in the hope of gaining protection against others. "You might have great immunity to flu x-y-z, but if it's p-d-q, you don't," he said.

ActiveStyle on 11/07/2016

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