Pandemic Influenza A (H1N1) During Winter Influenza Season in the Southern Hemisphere

Background: Countries in the southern hemisphere experienced sizable epidemics of pandemic influenza H1N1 in their winter season during May-August, 2009.

Methods: We make use of the Richards model to fit the publicly available epidemic data (confirmed cases, hospitalizations, and deaths) of six southern hemisphere countries (Argentina, Brazil, Chile, Australia, New Zealand, and South Africa) to draw useful conclusions, in terms of its reproduction numbers and outbreak turning points, regarding the new pH1N1 virus in a typical winter influenza season.

Results: The estimates for the reproduction numbers of these six countries range from a high of 1.53 (95% CI: 1.22, 1.84) for confirmed case data of Brazil to a low of 1.16 (1.09, 1.22) for pH1N1 hospitalizations in Australia. For each country, model fits using confirmed cases, hospitalizations, or deaths data always yield similar estimates for the reproduction number. Moreover, the turning points for these closely related outbreak indicators always follow the correct chronological order, i.e., case-hospitalization-death, whenever two or more of these three indicators are available.

Conclusions: The results suggest that the winter pH1N1 outbreaks in the southern hemisphere were similar to the earlier spring and later winter outbreaks in North America in its severity and transmissibility, as indicated by the reproduction numbers. Therefore, the current strain has not become more severe or transmissible while circulating around the globe in 2009 as some experts had cautioned. The results will be useful for global preparedness planning of possible tertiary waves of pH1N1 infections in the fall/winter of 2010.