Image caption The article warns that the H2N2 flu virus could re-emerge as a pandemic in the same way H1N1 did

Governments should launch a vaccination programme now to guard against a possible H2N2 flu pandemic, according to an article in the journal Nature.

The US authors say immunity to the H2N2 flu strain is very low in people under the age of 50.

But a safe vaccine already exists after an H2N2 outbreak in the 1950s and '60s.

They say that vaccinating now could save billions of dollars if a pandemic does develop.

Dr Gary Nabel and colleagues from the Vaccine Research Centre in the US say H2N2 has the ability to cause a pandemic in the same way that H1N1 did in 2009.

Between 1957 and 1968, the strain is thought to have caused up to 4 million deaths in a global outbreak, during which time a vaccine was developed.

When the pandemic was over the H2N2 vaccination programme was stopped in the late 1960s, although the virus is still present today among birds and swine.

The US Centres for Disease Control and Prevention estimates that a pandemic outbreak costs the United States between $71 billion and $167 billion Gary J Nabel, Vaccine Research Centre

Vulnerability of Youth

That means older people will have been vaccinated against the virus, but the relatively young will have missed out - what the authors call the vulnerability of youth.

Between 2003 and 2007 they examined levels of immunity to H2N2 among a small group of 90 people.

"Our study suggests that people under the age of 50 have little or no immunity, and resistance dramatically increases for those older than 50. This was also the case for the 2009 H1N1."

They argue that the vaccine developed in the 1950s would still work today and that governments should use this to develop a pre-emptive vaccination programme.

"One approach would be to manufacture the vaccine licensed in 1957 and immunise enough of the world's population to provide 'herd immunity' to the rest.

"This could be achieved by a 'one-time' campaign to immunise most of the adult population worldwide - for example, as part of standard seasonal flu vaccinations - accompanied by an ongoing programme to administer the vaccine to children."

Cheaper option

The authors say this would be a much cheaper option than stockpiling the vaccine or waiting for a pandemic to strike before boosting production.

Flu strains There are three types of seasonal influenza - A, B and C

Influenza A viruses are further divided into various subtypes, including H1N1, H1N2, H3N2 and H2N2

H1N1 appeared in 2009 and spread around the world, becoming a pandemic

Source: World Health Organization

"Another major influenza pandemic is likely to cost far more and create a much greater health burden than a well-planned pre-emptive programme.

"The US Centres for Disease Control and Prevention estimates that a pandemic outbreak costs the United States between $71 billion and $167 billion."

Dr Wendy Barclay, Chair in Influenza Virology at Imperial College, says the H2 flu virus does pose a credible pandemic threat, as do other strains of bird flu.

But she believes there are some big questions about whether a pre-emptive vaccination programme would be welcomed by the public.

"Now we are in the calm after the storm of that swine flu pandemic, it is timely to open up the debate about pre-pandemic vaccines. As Dr Nabel himself points out in his article, we have to ask whether the public will want or accept a vaccine against a disease that does not at the moment exist.

"Work towards making such vaccines available is ongoing in many laboratories around the world.

"Scientifically we are in a position to be able to offer a good solution, the issues to be decided are of cost and of public attitude."