Swine flu is behaving like the 1957 pandemic virus that killed about two million people worldwide, according to the first detailed scientific analysis of its spread.

Preliminary research into the H1N1 outbreak in Mexico indicated that the virus is slightly more dangerous than the one that caused a million deaths in the 1968 pandemic, but that it is nowhere near as deadly as the Spanish flu of 1918 that killed 50 million.

Transmission rates for swine flu are also similar to those of previous pandemics, with each person with the virus infecting between 1.4 and 1.6 others, the first study published by the World Health Organisation (WHO) Rapid Pandemic Assessment Collaboration found. The Mexican evidence indicates that the virus has a fatality rate of about 0.4 per cent.

The scientists have estimated that about 23,000 people contracted swine flu in Mexico before the end of April, when the WHO had confirmed only 257 infections worldwide, 97 of which were in Mexico. The number of confirmed cases rose yesterday to 4,694, including 53 deaths.

The findings were published in the journal Science, which expedited its peer-review procedures because of the significance for public health.

Professor Neil Ferguson, of Imperial College, London, who led the research, said it showed that the threat posed by the virus remained very real. The most similar historical precedent was the 1957 “Asian flu”, he told The Times. “We still don’t know how lethal it is, but very preliminary calculations for Mexico suggest it is a little worse than 1968 — it might be more like 1957,” he said.

“The worst-case scenario, which looks less likely from the UK and US data, is a pandemic about half as bad as 1918, which was of course catastrophic,” he said.

“Fortunately, it is looking like a relatively mild pandemic, but it is entirely likely that the burden of disease on the healthcare system is going to be substantially higher than an ordinary flu year. About 10 per cent of the population get sick with seasonal flu, and this could be on the order of two, perhaps three times worse than that if the early Mexican data are anything to go by.”

Although considerable uncertainties remained about every aspect of the virus, it was substantially more transmissible than seasonal flu and clearly had pandemic potential, he added.

The research was published as Keiji Fukuda, of the WHO, said that the virus could still start to spread in Europe as it had done in North America. “Whether it will develop exactly as it did in the United States and in Canada and in Mexico is anybody’s guess, but I think that the potential for it to be established either in Europe or in other places is there,” he said.

He added that there were still no signs of sustained community transmission of swine flu outside North America, which would have to be shown for a full pandemic to be declared. “The threshold for community spread is when you begin to see people who are getting infected, who become cases, and it’s just not clear where they’re getting it from,” he said.

In Britain, the Health Protection Agency confirmed a further 10 cases, bringing the total to 65. All but one had recently travelled to Mexico or the US, or had a close contact with a known infection, and the other case, from the east of England, remains under investigation.