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Infectiousness of swine flu estimated for New Zealand

Student sitting on lawn in front of the Clocktower

Friday 24 July 2009 11:20am

University of Otago, Wellington researchers say that the pandemic influenza virus appears more infectious than previously thought. The first estimate for New Zealand suggests that a person who is ill with this new strain will on average infect almost two others.

The transmission potential of a virus in a pandemic is typically summarised by the reproduction number. This measures 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 have detailed the latest estimate of the spread of the new virus in today's New Zealand Medical Journal.

This work was done in collaboration with Dr Hiroshi Nishiura, a mathematical modeller based at the University of Utrecht in Holland. It gives the first published estimate for the reproduction number of the pandemic in the Southern Hemisphere.

"Our best estimate of the reproduction number for the Influenza A virus in New Zealand is 1.96, which is somewhat higher than the number we have previously used in modelling estimates," says Associate Professor Baker. "To date we have tended to use a lower estimate of 1.5 which was published early in the pandemic based on data from Mexico."

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). However, that would require a high level of mixing between groups. Effective public health interventions could lower that proportion.

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

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

"This result reinforces the need for those with influenza symptoms to do everything they can to prevent infecting others," says Dr Nick Wilson. "Staying away from work and school while ill and practicing good 'cough etiquette' can all help to lower the spread of this new virus in New Zealand".

The letter to the New Zealand Medical Journal points out that the current estimated reproduction number is higher than that of the Spanish influenza pandemic of 1918-19, but the mortality rate is very low compared to that disaster, in which thousands died.

Earlier this month in a separate published study, Associate Professor Baker and Dr Wilson 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.

For further information contact

Associate Professor Michael Baker
University of Otago, Wellington
Tel 04 918 6802

Contents page for NZMJ:
PDF of the full document available on request.

The former publication on the case fatality ratio for this new strain is published at:

A list of Otago experts available for media comment is available elsewhere on this website.

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