Influenza seasonality: timing and formulation of vaccines.

Editorials 311 In all countries with temperate climates, a clear seasonal peak in influenza activity occurs during the winter months. 1 In countries with tropical or subtropical climates, however , influenza seasonality is more variable and influenza activity can be observed year-round – although it often is more intense during the rainy seasons. 2 Effective surveillance is essential for understanding influenza seasonality and for determining the best strategies for influenza control. The extensive influenza surveillance network that was established by the World Health Organization (WHO) during the 1950s has developed into the Global Influenza Surveillance and Response System. The tropical and subtropical Asian countries that participate in this system have recently expanded their influenza surveillance. This expansion has revealed not only that the patterns of influenza virus circulation in southern and southeastern Asia are complex but also that influenza activity in this area may show no marked seasonality.