Influenza epidemics, which can mainly be classified into A type and B type, occur throughout the world.  The activity of influenza has a clear and consistent seasonal distribution in the temperate regions with annual winter peaks.  In contrast, the temporal pattern of influenza is more variable in the tropical and subtropical regions, where multiple peaks can occur at different periods within a year.  To understand the seasonal variation of influenza activity in Hong Kong, the Observatory collaborated with the Microbiology Department of the Chinese University of Hong Kong to conduct a study on the relationship between weather and influenza activity in Hong Kong. The study analysed the information of the laboratory-confirmed influenza A and B cases admitted to the Prince of Wales Hospital and the Observatory's weather data recorded at Sha Tin from 1997 to 2006.

The study found that in general there were more influenza A than influenza B associated admissions in Hong Kong. The two types of influenza showed a different relationship with weather conditions. Influenza A had two seasonal peaks in Hong Kong which occurred respectively in winter/spring months (February to March) and summer months (June to July). This observation is in contrast with the situation in temperate regions where only a single winter peak is observed annually.  Influenza B also had a clear winter/spring peak, but its activity during the summer months was more variable.

It was also found that cold and humid conditions were associated with a higher level of activity of both influenza A and B in winter/spring months. In contrast, hot and humid conditions in summer months were associated with a higher level of activity of influenza A, but the association was not obvious for influenza B.

Furthermore, for influenza A, a shift in the relative magnitude of the two seasonal peaks was observed.  In the early part of the study period (1998-2000), the magnitude of the winter/spring peak of Influenza A was higher than the summer peak. Towards the latter part of the study period (2004-2006), the magnitude of the summer peak had become larger than the winter/spring peak (see figure).