Virologists led by Yoshihiro Kawaoka of the University of Wisconsin at Madison tested H1N1, taken from patients in the United States, as well as several seasonal flu viruses on mice, ferrets, macaque monkeys and specially-bred miniature pigs.

They found that H1N1 caused more severe lung lesions among mice, ferrets and macaques than the seasonal flu viruses.

But it did not cause any symptoms among the mini-pigs, which could explain why there has been no evidence that pigs in Mexico fell sick with the disease before the outbreak began among humans.

The team also found that, in experiments in lab dishes, the virus was highly sensitive to two approved and two experimental antiviral drugs, including Tamiflu, now being hurriedly stockpiled around the world.

This confirms the drugs' role as a "first line of defence" in the flu pandemic declared by the UN's World Health Organisation (WHO), they said.

The letter, published online by the British science journal Nature, said the swine flu virus appears to be related to a strain that unleashed the 1918 pandemic that killed tens of millions of people.

The evidence for this comes from blood samples from people born before 1920. Their blood had specific antibodies -- the immune system's frontline defence -- that recognised the new virus and responded to it.

Individuals born after 1920 did not appear to have antibodies capable of recognizing the new virus.

A similarity with the 1918 virus does not mean the viruses are equal in virulence, though.

Indeed, the present strain of swine flu virus is considered mild when compared with the pandemic viruses that erupted lethally several times in the last century.

According to scientists from the Massachusetts Institute of Technology (MIT) and the US Centers for Disease Control (CDC), the answer lies in the virus's poor ability to bind to a docking point on cells in the respiratory tract.

To do it more efficiently, it would have to undergo mutation in a key surface protein, they said in a paper published in the US journal Science on July 2.

A total of 94,512 cases of swine flu have been reported, including 429 deaths, according to the WHO's website on Monday, citing figures as of July 6.

Seasonal flu viruses are viruses that undergo only minor mutation and to which humans have a partial immunity. This explains why routine vaccines have to be tweaked every year, at the onset of the flu season, to provide a shield against a shifting adversary.

Pandemic viruses, though, are viruses against which there is no immunity, except among individuals who were exposed to a close cousin of the pathogen in the past.

The worry about the present strain of H1N1 is that it could pick up genes from other flu strains that would enable it to be both highly virulent and contagious.

These warnings are spelt out in the new study.

"Sustained person-to-person transmission might result in the emergence of more pathogenic variants, as observed in the 1918 pandemic virus," it says.

Another concern is that H1N1 could acquire mutations enabling it be resistant to Tamiflu.

"Collectively, our findings are a reminder that (strains of swine flu) have not yet garnered a place in history, but may still do so."

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