Worried about contracting the flu this year? Believe it or not, your risk may be linked to the year you were born. No, it's not astrology; it's simply a matter of the strain of flu you were exposed to as a child.

Health experts have long known that certain strains of flu are more likely to affect people of different ages. For example, while the very young and the very old are most at risk of contracting the flu, researchers have found that the H5N1 strain of the flu is most likely to affect children, but H7N9 seems to affect the older population. And now they finally know why.

According to a new study published in the journal Science, the year of your birth may affect the flu strain that is most likely to affect you. Researchers found that if you were exposed to the flu virus as a child, your body probably developed antibodies against that strain, making you less likely to contract that version of the flu in the future.

"As far as the data tell us, there is something kind of magical about the first time you have an influenza A response. It does seem to lock you into this imprinted immunity that you can benefit from," said Michael Worobey, a professor and head of the ecology and evolutionary biology department at the University of Arizona, who was a co-author of the study.

The flu timeline

So here's how it works. The average flu — Influenza A — can be broken down into two categories: strains H1, H2, and H5 are all "group one," while H3 and H7 are "group 2." By comparing historical data about flu epidemics with reported cases of the flu, researchers were able to figure out that people born before 1968 were likely exposed to different strains of flu viruses than those born after this year.

That's because there was a multinational flu pandemic in 1968 that affected people all over the world. The viruses involved were all group one viruses. After 1968, the flu strains that circulated among the general population transitioned to group two strains.

So people born in or before 1968 were exposed to either H1N1 or H2N2, both group 1 flu viruses. That makes these adults more vulnerable to H5 viruses (such as H5N1) later in life. On the flip side, those born after 1968 were more likely to be exposed to group two viruses, making them more susceptible to group one viruses as they get older.

Researchers found that a person's birth year not only determined what flu strains they were exposed to as children, it also resulted in a drastically lowered risk of contracting flu viruses from that same group later in life.

What makes things complicated is that for the last several years, the flu viruses that have been circulating in the U.S. are of the H1 and H3 varieties — so group one and group two. And health experts don't usually know which strain will dominate in any given year until flu season is well underway. (So yeah, you still need to get your flu shot.) But now researchers will have a better understanding about which age groups might be most at risk in a given year, and this can improve the way flu vaccines are made and the way that health experts respond to flu outbreaks.