Swine flu is more infectious than both seasonal influenza and the 1918 Spanish flu, a study based on New Zealand epidemic case numbers has found.
And this suggests nearly 79 per cent of an affected population will catch the virus, although not all will become ill.
The researchers, from Otago University at Wellington and Utrecht University in the Netherlands, say in a letter published in today's New Zealand Medical Journal that it was estimated early in the swine flu pandemic that the transmission rate was around 1.5. This means that every two people who are sick with the virus will infect three others.
This was calculated in Mexico, where the virus first emerged.
But Associate Professor Michael Baker, Dr Nick Wilson and Dr Hiroshi Nishiura estimate from New Zealand data that every case is infecting nearly two others - a rate of 1.96.
This is based on official notifications of swine flu between June 2 and 15. This was a period of exponential growth in the New Zealand epidemic. It began with the first case of local transmission, excludes 63 imported cases, and ends before health authorities, because so many people were catching the virus, gave up testing every potential case.
In Japan a transmission rate of between 2 and 2.6 was calculated.
The Medical Journal letter notes that a transmission rate of 1.96 is higher than published estimates for seasonal influenza in temperate-zone countries like New Zealand. It is also "slightly greater than that of Spanish influenza pandemic from 1918-19 in New Zealand", but the death rate from Spanish flu was much higher than in the current swine flu pandemic.
Drs Baker and Wilson have estimated the death rate from swine flu in developed countries could be as low as one death out of 10,000 cases.
The letter says a transmission rate of 1.96 suggests that 78.6 per cent of the population will experience infection by the end of the pandemic, although only two-thirds of those infected are expected to show symptoms.
But public health interventions like promoting good hygiene and social distancing were likely to reduce the proportion infected to below that estimate.
This is the first swine flu transmission rate estimate for the southern hemisphere. The researchers say the reasons it is higher than the Mexican estimate include that the south is now in winter and flu viruses spread more readily between people in wintertime.
The Ministry of Health said yesterday the official death toll from the swine flu pandemic stood at 11. However, chief coroner Judge Neil MacLean said he was investigating the deaths of a further three people who had the virus.
The ministry said the number of confirmed past and current cases - now just a fraction of the likely total number - was 2525.