Is natural immunity better than a flu vaccine? Is the immunity acquired by having the flu any more beneficial than immunity acquired via a flu shot?

A

Getting the flu itself can provide much stronger immunity than any flu shot does. But getting the flu is dangerous, so getting a flu shot is a better option.

When a flu virus enters your 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,” says Dr. Alicia Fry, a medical epidemiologist at the Centers for Disease Control and Prevention.

Studies also show that even when the antibody response wanes, immunity from a natural 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 will very likely make you sick. “When people become infected with the flu, they feel terrible. They miss school. They miss work. Some people end up in the hospital, and some people die,” says Dr. Fry. The risks are particularly concerning for older people, those with chronic diseases, pregnant women and young children. Even if you are healthy, you could put your family and others in your community at risk for infection.

Further, natural immunity to one flu virus won’t protect you 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,” says Dr. Fry. 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 it’s recommended you get a 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 immune response and how much of the virus you’re exposed to, according to Dr. Suryaprakash Sambhara, an immunologist at the C.D.C.

Flu vaccines use either an inactivated virus or a weakened live virus to mimic infection and spur a controlled immune response — but because the virus is weakened or inactivated, you don’t actually get the flu. Flu vaccines can prevent about 50 to 60 percent of flu infections, according to the C.D.C. Until recently both types of vaccines produced similar results. But over the last few years the live attenuated nasal spray (FluMist), which uses a weakened live virus, has been found to be ineffective and is not currently recommended by the C.D.C.

For perspective, a 2008 study published in Nature found that survivors of the 1918 Spanish flu got lifelong immunity to that flu virus and, as it turned out, protection against the pandemic H1N1 flu in 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, says it would be unwise to expose yourself to one flu virus in the hopes 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 says.