Divergent seasonal patterns of influenza types A and B across latitude gradient in Tropical Asia

INTRODUCTION: Influenza circulation in tropics and subtropics reveals a complex seasonal pattern with year-round circulation in some areas and biannual peaks in others.

METHODS: We analyzed influenza surveillance data from nine countries around southern and southeastern Asia spanning latitudinal gradient from equatorial to temperate zones to further characterize influenza type-specific seasonality in the region. We calculated proportion of positives by month out of positives during that year and adjust for variation in samples tested and positivity in these countries.

RESULTS: Influenza A epidemics were identified between November and March during winters in areas lying above 30°N latitude, during monsoon months of June-November in areas between 10° and 30°N latitude, and no specific seasonality for influenza A virus circulation in areas lying closer to the equator. Influenza B circulation coincided with influenza A circulation in areas lying above 30°N latitude; however, in areas south of 30°N Asia, influenza B circulated year round at 3-8% of annual influenza B positives during most months with less pronounced peaks during post-monsoon period.

CONCLUSION: Even though influenza B circulates round the year in most areas of the tropical regions of southern and southeastern Asia, the most appropriate time for influenza vaccination using the most recent WHO recommended vaccine would be prior to the monsoon season conferring protection against influenza A and B peaks.