Some people may already have a tiny bit of protection against H5N1 bird flu.
Several recent studies have found that those who caught seasonal flus or received flu vaccinations have low levels of antibodies against H5N1 avian influenza as well. The work may prompt changes to existing flu vaccines, potentially making them more effective against bird flu.
Health care workers and others who recently caught seasonal flu had antibodies that could glom onto H5N1 bird flu viruses taken from dairy cattle, researchers reported July 16 at bioRxiv.org. Some of those antibodies may block bird flu from infecting cells. Two strains of flu each account for roughly half of seasonal illnesses, but they affect potential H5N1 immunity differently. People who caught H1N1 seasonal flu had more antibodies aimed at bird flu than did those infected with the H3N2 strain.
Still, “everybody had relatively low levels of antibodies,” says Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health. Nine months post-infection, antibody levels had dipped even lower, he says. “The infection data probably is telling us that any boost in antibodies is going to be short lived.” This research has not yet been peer reviewed.
Many of those preexisting antibodies that grab H5N1 latch onto the protein hemagglutinin (the “H” in the names of these viruses), researchers reported March 13 in Nature Medicine. Vaccination against H1N1 could boost those antibodies, especially in young children who previously had few antibodies that could grab H5.
But Pekosz’s study, along with another posted July 16 to bioRxiv.org, suggest that some people also have antibodies directed against neuraminidase (the “N” in the name). Those results suggest that vaccine makers may want to focus more on building antibodies against that protein, Pekosz says. Antibodies that block neuraminidase probably wouldn’t guard against infection but may protect against severe disease, he says. “That could make a big public health impact at the beginning of a pandemic.”
Those studies couldn’t determine whether prior immunity can actually protect humans from H5N1, but research in ferrets hints that it might.
Recent seasonal flu infections protected ferrets against serious illness or death from an H5N1 bird flu virus, researchers reported July 23 in Science Translational Medicine. H5N1 is usually lethal for ferrets when given in high doses in the lab. But ferrets infected with either H1N1 or H3N2 seasonal strains a couple of months before getting H5N1 survived, says Penn State virologist Troy Sutton. “With H3N2 immunity, half the ferrets still got sick,” he says. But with H1N1 immunity, “ferrets didn’t get sick at all.”
In another experiment, researchers exposed ferrets with immunity to seasonal flu to another ferret carrying an H5N1 bird flu variant circulating in dairy cattle. “Preexisting immunity to H1N1, in particular, reduced the likelihood that the animal will get infected,” Sutton says.
Ferrets are often used as stand-ins for humans when studying flu and other respiratory viruses. But ferrets aren’t people, Sutton says. So, he and colleagues aren’t claiming that getting a seasonal flu will protect anyone against H5N1 bird flu.
None of the studies can predict whether H5N1 might cause a pandemic, nor can they forecast the severity of such a pandemic, says Jesse Bloom, a virologist and evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle who was not involved in any of the studies.
“Certainly, you can have pandemics even when there’s some existing weak immunity against the new strain,” he says. For instance, influenza pandemics in 1968 and 2009 happened despite many individuals having prior immunity to similar viruses. And preexisting immunity may have made the 2009 swine flu pandemic worse for some.
Even if preexisting immunity could shield some people from severe disease and death in a pandemic, it won’t protect everyone, Bloom says. “Probably an H5N1 pandemic is not going to kill everyone, but we should still be worried.”