Individuals receiving the annual vaccine for regular seasonal flu strains also develop protection against the more dangerous bird flu strain known as H7N9 that emerged in China 2 years ago, medical scientists in Chicago and New York have found.
Pharmacist putting bandage on patients arm after flu vaccination.
The seasonal flu vaccine may protect against bird flu.

The researchers, from the University of Chicago Medicine and the Icahn School of Medicine at Mount Sinai in New York, found antibodies – the immune system’s protective proteins – for H7N9 in people who had had the injection for seasonal flu. They have published their findings online in the Journal of Clinical Investigation.

Bird or avian flu virus emerged in China in 2013 and, as the researchers point out, shows a high death rate among people who become infected – H7N9 has killed a third of hospitalized patients.

While no pandemic has yet been seen, the threat of wider global spread is a worrying one, so the scientists – led by co-senior author Patrick Wilson, PhD, associate professor of medicine at the University of Chicago – looked to the regular seasonal vaccine, which produces antibodies against the flu virus, proteins that bind to the invading pathogen and neutralize it.

Dr. Wilson concludes that protection could be developed from the annual injection against H7N9:

“We have clear evidence that a normal immune response to flu vaccination offers protection against dangerous and highly unique strains of influenza such as H7N9. We now need to develop ways of amplifying this response.”

The researchers isolated antibodies from 28 people vaccinated with the regular seasonal injection:

  • They selected 83 antibodies that reacted with H3N2, a common human flu strain
  • Of these antibodies, 7% reacted against rare H7 strains
  • And of those, three antibodies appeared to completely neutralize H7N9 avian flu.

Flu vaccine-induced antibodies prevented bird flu deaths in mice

To verify their findings that the antibodies produced in flu-vaccinated people would protect against avian flu, the team treated mice with each antibody before exposing them to a lethal dose of H7N9 virus.

All three antibodies prevented death in the mice, whereas the untreated controls succumbed to infection.

The researchers also tested the antibodies as a therapeutic measure, administering them 24 hours after infection with the avian flu – once again, the mice were protected.

The study investigated the antibodies’ reactivity against wider flu strains, finding that H3 and other H7 strains could also be neutralized.

The researchers say the locations on the influenza virus that the antibodies bind to probably explain the broad reactivity. The regions are highly conserved, differing little between strains, they say.

Even if the virus’s mutations, which give it the notorious ability to work around new vaccines, occur at these conserved regions, the team found that the virus was significantly less infectious if antibodies bound to these sites.

Carole Henry, PhD, is a postdoctoral fellow at the University of Chicago and another of the study’s authors. Dr. Henry says: “It appears more common than previously thought for antibodies induced by flu vaccination to offer cross-protection against H7N9.” Dr. Henry adds:

Although they are not always protective, H7-reactive antibodies can be found in almost everyone that’s been vaccinated.”

The antibodies isolated by the team are effective but produced in “relatively low amounts” – and the researchers say “it is still unclear why.” They are working toward a better understanding of this, and to develop therapeutics based on the antibodies.

“The challenge is to exploit this response on a larger scale to make vaccines or therapeutics that offer broad protection against influenza strains,” Dr. Wilson says.

Meanwhile, he says “everyone should be vaccinated” because it is “clear” that seasonal vaccination “provides defense against more than just common strains.”

The potential threat of bird flu strains against humans will not be realized until they mutate and develop the ability to spread widely among us. Tracking the flu virus in chickens will help predict the potential for bird flu pandemics in humans, said Memphis and Beijing researchers in December.