Mystery immunity could boost swine flu protection

Children in Suining, Sichuan province, China have their temperature taken to check for symptoms of H1N1 influenza. An unexpected degree of immunity to the virus has developed. (Image: View China Photo/Rex Features)

• 17 September 2009 by Debora MacKenzie
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VACCINATING people against swine flu may be a lot easier than anyone dared hope, as it turns out that people have an unexpected degree of immunity to the pandemic now sweeping the world.

A seasonal flu in the same H1N1 family as the pandemic virus has been circulating since 1977, but until now it was thought that this seasonal virus did not induce immunity to the pandemic strain. This was because the pandemic virus spread faster than would be expected if there were widespread immunity to it, and because antibodies to the seasonal vaccine do not cross-react with it.

This looked like bad news for pandemic control. When people have no pre-existing immunity to a flu virus, they need not just one but two doses of vaccine several weeks apart - making it very hard to immunise many people in time to protect them. But last week the Swiss firm Novartis and the Australian firm CSL reported in The New England Journal of Medicine (DOI: 10.1056/nejmoa0907413) that nearly 300 adults given their experimental pandemic vaccines "unexpectedly" developed protective antibodies after just one shot. The Chinese vaccine firm Sinovac and the French firm Sanofi report similar results.

"Somehow people's immunity has been primed," says Michael Greenberg of CSL. His team found antibodies that reacted with the pandemic virus in 1 in 3 of the people tested. The findings suggest that seasonal flu vaccine boosts those antibodies slightly - perhaps a reason to get a shot this year.

These antibodies can't be the whole story, though, because people without them also responded swiftly to the vaccine. Seasonal H1N1 infection may have primed another part of the immune system, called cell-mediated immunity, which may not prevent infection but limits the severity of the disease. This could be why the pandemic has spread fast but remained mild in many people, though not all.

More good news comes from computer modellers at the University of Western Australia, Crawley. They have calculated that "priming" people before a pandemic begins with an initial dose of vaccine, even if it doesn't match the pandemic virus exactly, is more effective than waiting to give two doses of the correct vaccine after a pandemic starts (Journal of the Royal Society Interface, DOI: 10.1098/rsif.2009.0312). The immunity that many people now have has the same effect as this priming, so that just one shot of vaccine could result in full immunity.





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