The MRC Centre for Outbreak Analysis and Modelling at Imperial College London and the World Health Organisation have worked with public health agencies in Mexico, to assess the ongoing swine flu epidemic. Their results are published today in Science and can be summarized:

• In Mexico, influenza A (H1N1) is fatal in about 4 in 1000 cases, which is comparable to the mortality rate in the 1957 pandemic.
• The 2009 H1N1 outbreak started in Mexico on 15 February 2009. 23,000 people there had been infected by the end of April and there were 91 deaths due to infection. These are not definitive numbers, infection rate could be between 6,000 and 32,000.
• This uncertainty means case fatality ratio (CFR) could be as low as 0.3% or as high as 1.5%; researchers believe 0.4% is best guesstimate.
• For every person infected, there may be 1.2-1.6 secondary cases on average, which is higher than normal seasonal influenza, but a lower rate than “normal” pandemic influenza.
• Childhood infection rate seems to be higher, suggesting adults have some immunity. Alternatively, children may just be exposed more because of mixing at school.

MRC’s Neil Ferguson, says, “Our study shows that this virus is spreading just as we would expect for the early stages of a flu pandemic…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.”

Christophe Fraser (2009). Pandemic Potential of a Novel Strain of Influenza A (H1N1): Early Findings Science