Birth year may affect your flu risk. Here's how

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People's birth years can affect their risk of catching certain strains of influenza — probably because their first case of flu somehow sets their immune system, researchers reported Thursday.

Their findings could be good news for what scientists predict about the risk of a killer flu pandemic, and they could also help researchers find better flu vaccines, the researchers said.

"Our work implies that we have never seen a true 'virgin soil' influenza pandemic," the team wrote in their report, published in the journal Science. "Virgin soil" means a population that has no immunity at all to a new infection.

The teams were looking at the puzzling characteristics of two strains of bird flu that keep popping up in people: the H5N1 and H7N9 avian influenza viruses.

They've been spilling over into people for years and scaring scientists who see the potential for a pandemic of flu that could kill tens or even hundreds of millions of people. But this hasn't happened yet.

And, oddly, while H5N1 has hit young people and children especially hard, H7N9 seems to attack older people.

RELATED: History of flu outbreaks

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History of bird flu outbreaks
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History of bird flu outbreaks
This undated handout image provided by Science and the University of Tokyo shows infectious particles of the avian H7N9 virus emerging from a cell. Scientists who sparked an outcry by creating easier-to-spread versions of the bird flu want to try such experiments again using a worrisome new strain. Since it broke out in China in March, the H7N9 bird flu has infected more than 130 people and killed 43. Leading flu researchers say that genetically engineering this virus in the lab could help track whether itâs changing in the wild to become a bigger threat. They announced the pending plans Wednesday in letters to the journals Science and Nature. (AP Photo/Takeshi Noda/University of Tokyo, Science)
Thomas 'Tom' Vilsack, secretary of the U.S. Department of Agriculture (USDA), speaks during an interview in Washington, D.C., U.S., on Monday, June 8, 2015. Government spending to fight the worst U.S. bird flu outbreak and compensate farmers for their losses will exceed the $410 million so far budgeted and may top a half-billion dollars, Vilsack said. Photographer: Andrew Harrer/Bloomberg via Getty Images
FILE - In this Nov. 16, 2009 file photo, chickens stand in their cages at a farm near Stuart, Iowa. Discovery of the bird flu on an Iowa turkey farm has raised serious concerns that the bird killer could find its way into chicken barns in the nation’s top egg-producing state and rapidly decimate the flocks that provide the U.S. with its breakfast staple. (AP Photo/Charlie Neibergall, File)
Workers in protective clothing work at the Daybreak Foods Inc. hen farm in Jefferson County near Lake Mills, Wis., Friday, April 24, 2015. There are two avian flu outbreaks in Jefferson county. The virus is lethal to birds, but is not expected to be a risk to people or the food supply. (AP Photo/Andy Manis)
Workers in protective clothing work at a hen farm in Jefferson County near Lake Mills, Wis., Friday, April 24, 2015. There are two avian flu outbreaks in Jefferson county. The virus is lethal to birds, but is not expected to be a risk to people or the food supply. (AP Photo/Andy Manis)
Workers in protective clothing work at the Daybreak Foods Inc. hen farm in Jefferson County near Lake Mills, Wis., Friday, April 24, 2015. There are two avian flu outbreaks in Jefferson county. The virus is lethal to birds, but is not expected to be a risk to people or the food supply. (AP Photo/Andy Manis)
Indian workers spread disinfectant after an operation to cull chickens at Venkateshwara Hatcheries in Thoroor village in Ranga Reddy district, some 55 kilometers from Hyderabad on April 15, 2015. Five chicks were found to be infected with H5N1 avian influenza on regular testing of samples belonging to the farm of a poultry farmer Srinivas Reddy. The authorities ordered the culling of 150,000 birds in a kilometre radius on poultry farms, although no cases of human infections were identified so far, according to Ranga Reddy district officials. AFP PHOTO / Noah SEELAM (Photo credit should read NOAH SEELAM/AFP/Getty Images)
Indian workers spread disinfectant after an operation to cull chickens at Venkateshwara Hatcheries in Thoroor village in Ranga Reddy district, some 55 kilometers from Hyderabad on April 15, 2015. Five chicks were found to be infected with H5N1 avian influenza on regular testing of samples belonging to the farm of a poultry farmer Srinivas Reddy. The authorities ordered the culling of 150,000 birds in a kilometre radius on poultry farms, although no cases of human infections were identified so far, according to Ranga Reddy district officials. AFP PHOTO / Noah SEELAM (Photo credit should read NOAH SEELAM/AFP/Getty Images)
Indian health workers dump bags of dead chickens after a culling operation at Venkateshwara Hatcheries in Thoroor village in Ranga Reddy district, some 55 kilometers from Hyderabad on April 15, 2015. Five chicks were found to be infected with H5N1 avian influenza on regular testing of samples belonging to the farm of a poultry farmer Srinivas Reddy. The authorities ordered the culling of 150,000 birds in a kilometre radius on poultry farms, although no cases of human infections were identified so far, according to Ranga Reddy district officials. AFP PHOTO / Noah SEELAM (Photo credit should read NOAH SEELAM/AFP/Getty Images)
Indian health workers carry dead chickens in bags after a culling operation at Venkateshwara Hatcheries in Thoroor village in Ranga Reddy district, some 55 kilometers from Hyderabad on April 15, 2015. Five chicks were found to be infected with H5N1 avian influenza on regular testing of samples belonging to the farm of a poultry farmer Srinivas Reddy. The authorities ordered the culling of 150,000 birds in a kilometre radius on poultry farms, although no cases of human infections were identified so far, according to Ranga Reddy district officials. AFP PHOTO / Noah SEELAM (Photo credit should read NOAH SEELAM/AFP/Getty Images)
Dead chicken, right, lie at a poultry farm in Katmandu, Nepal, Thursday, Aug. 1, 2013. A Nepalese official said the government has banned the sale and transport of chicken and all poultry products in the capital city to prevent the spread of the H5N1 bird flu virus. Agriculture Ministry spokesman Prabhakar Pathak said Thursday that the virus has been detected in several poultry farms in Katmandu and surrounding areas. No human casualties have been reported. (AP Photo/Bikram Rai)
In this April 13, 2014 photo provided by Kumamoto Prefecture, chickens are seen at a farm where H5 virus was detected in two birds on Sunday, in Taragicho, western Japan. The 112,000 chickens were ordered culled on Monday, April 14 after the two tested positive for a highly pathogenic avian influenza in the town. (AP Photo/Kumamoto Prefecture)
In this Tuesday, Jan. 21, 2014 photo, live chickens are kept in a cage at a wholesale poultry market in Shanghai. A spate of bird flu cases since the beginning of the year in China has experts watching closely as millions of people and poultry are on the move ahead of the Lunar New Year holiday, the world's largest annual human migration. (AP Photo)
Ans Hermans throws her chickens in their air, releasing them after more than two months indoors, in Nunhem, south east Netherlands, Monday, May 1, 2006. The Netherlands' Agriculture Ministry lifted an order keeping all domestic poultry indoors, as fears over an outbreak of birdflu eased. (AP Photo/ Ermindo Armino)
Hens are seen inside a chicken farm in Baexem, south-east Netherlands, Thursday, Feb. 16, 2006. After two dead swans in neighbouring Germany had been preliminarily tested positive for the deadly H5N1 bird flu strain, the Dutch agriculture ministry Wednesday urged commercial poultry farmers to get their birds indoors as soon as possible as a protective measure to prevent an outbreak of bird flu. (AP Photo/John Peters)
Poultry farmer Vermeij feeds his chickens inside his chicken farm in Baexem, south-east Netherlands, Thursday, Feb. 16, 2006. After two dead swans in neighbouring Germany had been preliminarily tested positive for the deadly H5N1 bird flu strain, the Dutch agriculture ministry Wednesday urged commercial poultry farmers to get their birds indoors as soon as possible as a protective measure to prevent an outbreak of bird flu. (AP Photo/John Peters)
Ans Hermans throws her chicken in the air, releasing it after more than two months indoors, in Nunhem, south east Netherlands, Monday, May 1, 2006. The Netherlands' Agriculture Ministry lifted an order keeping all domestic poultry indoors, as fears over an outbreak of birdflu eased. (AP Photo/ Ermindo Armino)
Hens are seen inside a chicken farm in Baexem, south-east Netherlands, Thursday, Feb. 16, 2006. After two dead swans in neighbouring Germany had been preliminarily tested positive for the deadly H5N1 bird flu strain, the Dutch agriculture ministry Wednesday urged commercial poultry farmers to get their birds indoors as soon as possible as a protective measure to prevent an outbreak of bird flu. (AP Photo/John Peters)
A shelduck receives vaccination against Bird Flu at Blijdorp Zoo in Rotterdam, the Netherlands, Tuesday, Dec. 13, 2005. The zoo is believed to be the first in Europe to begin inoculation against the H5 strains of bird virus, including the H5N1 strain that has swept through flocks and killed at least 69 people since 2003. (AP Photo/Fred Ernst)
Hens are seen inside a chicken farm in Baexem, south-east Netherlands, Thursday, Feb. 16, 2006. After two dead swans in neighbouring Germany had been preliminarily tested positive for the deadly H5N1 bird flu strain, the Dutch agriculture ministry Wednesday urged commercial poultry farmers to get their birds indoors as soon as possible as a protective measure to prevent an outbreak of bird flu. (AP Photo/John Peters)
Dutch State Secretary Henk Bleker (C) puts on a mask during a visit to a turkey farm affected by a bout of the bird flu virus in Kelpen-Oler on March 19, 2012. All 42,700 turkeys at the farm will be slaughtered. AFP PHOTO / ANP MARCEL VAN HOORN netherlands out (Photo credit should read MARCEL VAN HOORN/AFP/Getty Images)
Chart shows the number of bird’s with bird flu since March; 2c x 3 inches; 96.3 mm x 76 mm;
This photo provided by the Iowa Department of Natural Resources shows chickens in a trench on a farm in northwest Iowa. Millions of dead chickens and turkeys are decomposing in fly-swarmed piles near dozens of Iowa farms, culled because of a bird flu virus that swept through the state's large poultry operations. (Iowa Department of Natural Resources via AP)
EAGLE GROVE, IA - MAY 17: A gate blocks the entrance of a farm operated by Daybreak Foods which has been designated 'bio security area' on May 17, 2015 near Eagle Grove, Iowa. Daybreak Foods is one of several large-scale commercial poultry facilities is Iowa reported to have been hit with a highly pathogenic strain of avian influenza which has forced poultry producers to kill off millions of birds in an attempt to stifle the spread of the illness. A road leading up to the front of the farm has been closed to outside traffic with a checkpoint established. (Photo by Scott Olson/Getty Images)
Nick Wells puts eggs in a cooler at the Waveland Cafe, Friday, June 19, 2015, in Des Moines, Iowa. Restaurants are struggling to deal with higher egg prices and an inability to get enough eggs and egg products in the midst of a shortage brought about by a bird flu virus that wiped out millions of chickens on commercial farms this spring.(AP Photo/Charlie Neibergall)
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H5N1 has infected 856 people since 2003 and killed 452 of them, WHO reports. H7N9 has infected 452 people and killed 124 of them, according to the Center for Infectious Disease Research and Policy at the University of Minnesota.

The team at the University of Arizona and the University of California Los Angeles dove into the numbers, examining the reports of each case, who was affected and how severely.

Related: Where has H5N1 Been, Anyway?

They found an almost startling link with birth year.

"It's ridiculously predictive," said Michael Worobey, an expert in viral genetics at the University of Arizona.

Worobey thinks he knows why. Other research suggests that early infections "imprint" the body's immune system. It makes sense — the human immune system makes cells and antibodies that "learn" to recognize and react more quickly to microbial invaders the next time they infect someone.

Worobey has also found that influenza viruses that commonly infect people can be categorized into two groups. Group 1 consists of subtypes H1, H2, and avian H5, while group 2 includes seasonal H3 and avian H7.

Related: What's in a Flu Name?

Influenza A viruses all are named for two proteins found on their surfaces: hemagglutinin (the H in a flu name) and neuraminidase (the N in a flu name). Thus flu strains get names such as H5N1 or H1N1 or H3N2. So human H1N1, H2N2 and the H5N1 bird flu virus are all related in Group 1, while the H3N2 seasonal flu virus is related to the H7N9 bird flu strain.

Every few decades, the predominant flu strain changes. So the H1N1 flu strain that killed tens of millions of people in 1918 hung around infecting people until 1957, when a pandemic of H2N2 flu swept the world and took over. Then H2N2 predominated until 1968, when the H3N2 "Hong Kong" flu took over.

Most recently, the H1N1 "swine flu" caused a minor pandemic in 2009. Now H1N1 and H3N2 both regularly circulate as seasonal flu viruses.

What Worobey and colleagues found was that people born in 1968 and later are less likely to be made severely ill by H7N9. They believe this is because the first flu strain that would have infected them would have been H3N2, which is in the same group as H7N9.

Related: Canadian Woman Infected With H7N9 Flu

And people born before 1968 are less likely to die or be made severely ill by H5N1, because their first flu infection would have been an H1N1 or H2N2 strain.

"Roughly half of the people are quite well-protected against one of these potential pandemic viruses but not the other," Worobey said.

"Our findings show clearly that this 'childhood imprinting' gives strong protection against severe infection or death from two major strains of avian influenza," added James Lloyd-Smith, a UCLA professor of evolutionary biology who worked on the study. "These results will help us quantify the risk of particular emerging influenza viruses sparking a major outbreak."

"It's imprinting. Not the virus, not the behavior of people, but being exposed to different viruses at an early age," Worobey said.

Flu strains mutate all the time, which is why people need a fresh flu shot every year. But Worobey and other researchers say there are unchanging, or "conserved" parts of the flu virus on the "stem" of the mushroom-shaped hemagglutinin structure that the human immune system can recognize.

It obviously does not provide perfect protection, but enough to protect most people with this immune imprinting from the most severe disease, he said.

"It is an extra barrier that is probably protecting us from even more frequent influenza pandemics," he said.

More importantly, later flu infections do not seem to alter this mechanism, although this theory needs testing.

The researchers cannot say whether regular flu vaccination affects this "imprinting" effect. People are now encouraged to get annual flu vaccines starting as infants, and they get a cocktail of vaccines that protect against both the Group 1 H1N1 and Group 2 H3N2 viruses.

They also cannot say whether peoples' susceptibility to seasonal flu is affected by imprinting. Flu is so very common that it will be hard to say which virus first infected someone and then follow them through life to see which strains make them sicker.

But it might be possible to use the findings to make better vaccines, Worobey said.

"This is really the first evidence out in the real world that this kind of cross-protection is possible," Worobey said.

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