Did you get your flu shot? If so, you probably figured it would protect you against exposure to the three strains of influenza virus deemed most likely to circulate this winter. But a new study in the journal Science reveals that the shot primed your immune system to defeat a much broader range of flu viruses, and it suggests a simple tweak to make it work even better.

Every year, 5 to 10 percent of adults and 20 to 30 percent of kids around the world become sick with the flu, the World Health Organization said. In 3 million to 5 million cases, the flu can cause more severe illness than a week of painful muscles and a runny nose, including 250,000 to 500,000 deaths.

That's why virologists put so much effort into developing the flu vaccine. It contains samples of weakened or deactivated viruses, enough to make the immune system swing into action and create antibodies. That way, if you encounter a real flu virus, your body will be prepared to fight it.

But influenza is a wily virus. There are three main types — A, B and C — and hundreds of subtypes. And these strains are constantly changing. Experts at public health agencies such as the WHO and the U.S. Centers for Disease Control and Prevention track the ones circulating in people and make their best guess about the three or four versions that are most likely to cause infection and put those strains in the flu shot (or flu mist). The better the match, the more protection, and vice versa.

But the new study in Science reported that an international research team found that the flu shot prompts the body to make antibodies against all of the flu viruses a person has encountered over his or her lifetime — not just the ones in the vaccine. In fact, the immune response "tends to be highest against strains encountered earlier in life," they wrote.

The researchers came up with a name for this phenomenon: a "back-boost." Their first sign of its existence came from studying the blood samples of 69 people in Vietnam who were born between 1917 and 2005 and tracked for another study. All of them had given blood samples each year between 2007 and 2012. None had gotten flu shots, but they had been exposed to the flu (and sometimes got sick).

The researchers used the blood samples to create 3-D computer models that they called "antibody landscapes." These maps depicted the relationships among various flu strains, along with the strength of the antibody response to each one.

What these landscapes showed was that the strongest immune response was usually to flu viruses that were circulating when people were about 6 years old. The upshot, researchers said, was when designing a flu shot, you're better off including new strains instead of old ones. Either way, you get protection against older flu strains. And if you guess right, you'll get protection for new ones as well.

Los Angeles Times