Birth year may help predict your risk in next flu outbreak: study

Your birth year may help predicts, to a certain extent, how likely you are to get seriously ill or die in an outbreak of an animal-origin flu virus similar to H5N1 or H7N9, U.S. researchers said Thursday.

Until now, it was believed that previous exposure to a flu virus offered people little or no immunological protection against new flu viruses that can jump from animals into humans, according to the study co-led by researchers from the University of Arizona (UA) in Tucson and the University of California, Los Angeles.

But after analyzing data from every known case of severe illness or death caused by the H5N1 and H7N9 bird flu viruses, they discovered that the first infection with flu virus as a child determines which new bird flu strains they would be protected against in a future infection.

When an individual gets exposed to flu virus for the first time, the immune system makes antibodies targeting hemagglutinin, a receptor protein shaped like a lollipop that sticks out from the virus surface, they explained.

Like lollipops that come in different colors and flavors, flu viruses differ from each other in the parts that make up their hemagglutinins.

But each of the 18 known influenza A virus hemagglutinin subtypes falls into one of just two main "flavor" groups. One group consists of human H1 and H2 viruses as well as avian H5, while the other contains human H3 and avian H7.

"In this analogy, let's say you were first exposed to a human 'orange lollipop' flu as a kid," said Michael Worobey, head of the UA's Department of Ecology and Evolutionary Biology and one of the two senior authors of the study.

"If later in life you encounter another subtype of flu virus, one from a bird and one that your immune system has never seen before but whose proteins also are of a similar 'orange' flavor, your chances of dying are quite low because of cross-protection. But if you were first infected with a virus from the 'blue lollipop' group as kid, that won't protect you against this novel, 'orange' strain."

For people who were born before the late 1960s and exposed to "blue lollipop" flu as children, H1 or H2, they rarely succumb to avian H5N1 -- which shares a "blue" hemagglutinin -- but often die from "orange" H7N9, according to the study published in the U.S. journal Science.

For people who were born after the late 1960s and exposed to "orange lollipop" flu as children, H3, they are protected from H7N9 but suffer severe disease and death when exposed to H5 viruses.

The study also estimated that there is a 75 percent protection rate against severe disease and 80 percent protection rate against death if patients had been exposed to a matched virus as children.

Based on these findings, Worobey said future research should try to elucidate the exact mechanism underlying the so-called "immunological imprinting" and finding out possible ways to modify it with a vaccine.

"In a way it's a good-news, bad-news story," he said. "It's good news in the sense that we can now see the factor that really explains a big part of the story: Your first infection sets you up for either success or failure in a huge way, even against 'novel' flu strains."

"The bad news is the very same imprinting that provides such great protection may be difficult to alter with vaccines: A good universal vaccine should provide protection where you lack it most, but the epidemiological data suggest we may be locked into strong protection against just half of the family tree of flu strains." Enditem