Scientists say each person with swine flu will infect two others

Scientists in New Zealand who have been examining the spread of the new H1N1 flu (swine flu), say the virus appears more infectious than previously thought and a person who becomes ill with the new strain will on average infect almost two others.

Their research which is the first estimate for New Zealand suggests that almost 80% of the population could catch the virus.

The researchers from the University of Otago in Wellington say that the transmission potential of a virus in a pandemic is typically summarised by the reproduction number which indicates the average number of secondary cases generated by a single primary case.

Associate Professor Michael Baker and Dr. Nick Wilson from the Department of Public Health at the University of Otago, Wellington worked in collaboration with Dr. Hiroshi Nishiura, a mathematical modeller based at the University of Utrecht in Holland and their results offer the first published estimate for the reproduction number of the pandemic in the southern hemisphere.

Associate Professor Baker says their best estimate of the reproduction number for the Influenza A virus in New Zealand is 1.96, which is higher than the number previously used in modelling estimates, where a lower estimate of 1.5 was used early in the pandemic based on data from Mexico.

The researchers say a reproduction number of 1.96 means that up to 79% of an affected population could ultimately catch H1N1 influenza during the epidemic, though only two-thirds of those infected may be expected to show symptoms - but this would require a high level of mixing between groups and they say effective public health interventions could lower that proportion.

Published estimates of the reproduction number in the current influenza pandemic have ranged from 1.4-1.6 in Mexico to 2.0-2.6 in Japan and it is thought that New Zealand has a higher reproduction number than Mexico, because of the winter season and large clusters of cases in certain settings.

The researchers say that further work will be needed to produce more robust estimates for New Zealand and this latest estimate could be reduced after more accurate demographic analysis, and if public hygiene behaviour and health interventions are effective.

Dr. Wilson says the results reinforce the need for those with influenza symptoms to do everything they can to prevent infecting others - by staying away from work and school while ill and practicing good 'cough etiquette' - all of which will help to lower the spread of the new virus.

The team say though the current estimated reproduction number is higher than that of the Spanish influenza pandemic of 1918-19, the mortality rate in this pandemic is very low compared to that disaster, in which thousands died.

Earlier this month in a separate published study, the two New Zealand scientists estimated that the mortality rates in developed countries from this pandemic could be as low as one death out of 10,000 cases for this new pandemic strain.

The research is published in the New Zealand Medical Journal.