The viral strain that sparked off the swine flu in Mexico, A(H1N1), may be as deadly as the one found in the 1957 pandemic.
Imperial College London researchers came to this conclusion after analysing the pandemic potential of swine flu in collaboration with the World Health Organisation and public health agencies in Mexico.
The researchers' best estimate is that in Mexico, influenza A (H1N1) is fatal in around 4 in 1,000 cases, which suggests that it may be as lethal as the influenza strain found in the 1957 pandemic.
The epidemic of influenza A (H1N1) presumably started in Mexico on February 15, and the data suggests that by the end of April, around 23,000 people were infected with the virus in Mexico. The researchers point out that 91 of those died as a result of infection.
However, the figures are uncertain because some mild cases might have gone unreported.
According to the researchers, the numbers infected could be as low as 6,000 people or as high as 32,000 people.
They say that the uncertainty around the numbers of people who have been infected with influenza A (H1N1) in Mexico means that the case fatality ratio (CFR) of 0.4 per cent, that is 4 deaths per 1000 infected persons, cannot be definitely established.
While the CFR is currently in the range of 0.3 to 1.5 per cent, the researchers believe that 0.4 per cent is the most likely.
The team say for every person infected, it is likely that there will be between 1.2 and 1.6 secondary cases, which is high as compared to normal seasonal influenza in which around 10-15 per cent of the population are likely to become infected.
However, it is lower than would be expected for pandemic influenza, where 20-30 percent of the population are likely to become infected.
Analysing an outbreak in an isolated village called La Gloria in Mexico, the researchers also observed that children were twice as likely to become infected as adults, with 61 per cent of those aged under 15 becoming infected, compared with 29 per cent of those over 15.
Based on that observation, the researchers surmise that adults have some degree of immunity against infection because of having been previously infected with a related strain of influenza, or it may mean that children are more susceptible to infection because they interact much more closely together, such as in school, than adults.
Professor Neil Ferguson, the corresponding author of today's research from the MRC Centre for Outbreak Analysis and Modelling at Imperial College London, said:
"Our study shows that this virus is spreading just as we would expect for the early stages of a flu pandemic. So far, it has been following a very similar pattern to the flu pandemic in 1957, in terms of the proportion of people who are becoming infected and the percentage of potentially fatal cases that we are seeing."
Furguson added: "What we're seeing is not the same as seasonal flu and there is still cause for concern - we would expect this pandemic to at least double the burden on our healthcare systems. However, this initial modelling suggests that the H1N1 virus is not as easily transmitted or as lethal as that found in the flu pandemic in 1918," added Professor Ferguson." he viral strain that f