Governments must start to vaccinate children and young adults against a strain of the influenza virus that killed several million people in the 1960s, a leading expert has warned.

The last outbreak of the H2N2 flu occurred in 1968 but the virus is now circulating in birds and pigs and could easily cross into the human population where most people under the age of 50 have little or no immunity, according to Gary Nabel, a vaccine researcher at the US National Institutes of Health in Bethesda, Maryland.

A pre-emptive vaccination campaign targeted at children and young people could help to avert a major pandemic. The last epidemics of H2N2 occurred between 1957 and 1968 and are estimated to have killed between one million and four million people.

The 2009 flu pandemic, caused by the H1N1 strain of the virus, caught the world by surprise as it bore a remarkable similarity to the 1918 strain that resulted in the worst flu pandemic in history, with 50 million deaths. Until then, experts had expected that the next pandemic would result from a re-shuffling of the genes of a seasonal flu strain and one that had not circulated before, Dr Nabel said.

"This unexpected source of the 2009 H1N1 pandemic is a cautionary tale. Another subtype of influenza, H2N2, looms as a public health threat and could re-emerge in a similar way," Dr Nabel and his colleagues write in the journal Nature.

"Governments, regulatory agencies and industry should develop a pre-emptive vaccination programme for H2N2. Waiting until an outbreak occurs risks making the same kind of mistakes made over the H1N1 pandemic."