The H1N1 swine flu that swept the globe in 2009/10 could easily morph into a more transmissible form, while an older, mid-20th century virus could also come roaring back, scientists warned in separate studies.

The so-called Asian influenza, a H2N2 strain, first appeared in 1957 and killed one to four million people despite a major vaccination campaign.

Studies have shown that most people today aged 50 or older retain some immunity to the virus, which continues to circulate in birds and swine.

Those younger, however, are acutely vulnerable to H2N2, which increases the chance that the potentially lethal strain will jump back into humans and once again spread across the world.

“H2N2 looms as a public health threat, and could re-emerge,” Gary Nobel, a researcher at the Vaccine Research Centre of the US. National Institutes of Health and lead author of a commentary published Thursday in Nature.

Governments, the WHO and drug companies “should develop a pre-emptive vaccination programme,” he urged. Dr Nobel outlines three strategies for anticipating a resurgence of the bird-borne virus.

One is to manufacture the same vaccine licenced in 1957 and immediately inoculate enough of the world’s population to provide what scientists call a “herd immunity” to the rest. Alternatively, health authorities could stockpile the same vaccine in the event of an outbreak, or make “master lots” and ramp up production at the first sign of an outbreak. Acting now would be far more cost effective than waiting for the strain to reappear, Dr Nobel argued, citing a US government study estimating that a flu pandemic costs the US $71 to $167 billion.