Work to be published soon, by John Oxford of Barts and The London School of Medicine and Dentistry, also shows that infected volunteers with high cell-mediated immunity get few or no flu symptoms.

However, infections also trigger another reaction called cell-mediated immunity, in which certain white blood cells target and destroy infected cells.

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 .