There has been much discussion of the effect of the weather on flu transmission. In general, the expectation is that flu spreads more effectively during the winter. Does this pattern apply to the new H1N1 virus? We know that the virus is continuing to spread and kill in the US, Canada and the UK during the Northern Hemisphere summer. Although countries in the Southern Hemisphere appear to be disproportionately affected, individual differences in the reporting of cases among countries confounds attempts to compare them.
The ideal test case for the seasonality hypothesis would be a single country that spans different climatic zones. Brazil fulfills this criterion. This large country spans a range from just north of the equator to 35 degrees south latitude. Previous studies have demonstrated a clear seasonal pattern of mortality associated with latitude:
From Alonso, et al. (2007) Seasonality of influenza in Brazil: A Traveling Wave from Amazon to the Subtropics. American Journal of Epidemiology. 165: 1434-1442.
Seasonality in mortality was most pronounced in southern states (winter epidemics, June–July), gradually attenuated toward central states (15°S) (p < 0.001), and remained low near the equator.
Laboratory surveillance data from recent years provided independent confirmation that mortality peaks coincided with influenza virus activity. The direction of the traveling wave suggests that environmental forces (temperature, humidity) play a more important role than population factors (density, travel) in driving the timing of influenza epidemics across Brazil.
There have been at least 56 deaths due to the new H1N1 in Brazil (and likely more). 27 have been reported in state of São Paulo, 19 in the state of Rio Grande do Sul, 5 in Rio de Janeiro, 4 in Paraná and 1 in Paraiba. With the exception of Paraiba, all these states are located in the southern part of Brazil. Thus, the pattern of infections of the new H1N1 within Brazil is similar to that seen with seasonal flu: it is found predominantly in the southern part of the country in June and July, suggesting a strong effect of weather on transmission.
Although the new H1N1 virus has a remarkable ability to spread in the summer in the Northern Hemisphere, we have every reason to believe that the spread will be even more ferocious in the fall and winter.