Introduction
Determining the optimal time to vaccinate is important for influenza vaccination programmes.
Here, we assessed the temporal characteristics of influenza epidemics in the
Northern and Southern hemispheres and in the tropics, and discuss their implications for
vaccination programmes.
Methods
This was a retrospective analysis of surveillance data between 2000 and 2014 from the
Global Influenza B Study database. The seasonal peak of influenza was defined as the
week with the most reported cases (overall, A, and B) in the season. The duration of seasonal
activity was assessed using the maximum proportion of influenza cases during three
consecutive months and the minimum number of months with 80% of cases in the season.
We also assessed whether co-circulation of A and B virus types affected the duration of
influenza epidemics.
Results
212 influenza seasons and 571,907 cases were included from 30 countries. In tropical
countries, the seasonal influenza activity lasted longer and the peaks of influenza A and B
coincided less frequently than in temperate countries. Temporal characteristics of influenza
epidemics were heterogeneous in the tropics, with distinct seasonal epidemics observed
only in some countries. Seasons with co-circulation of influenza A and B were longer than
influenza A seasons, especially in the tropics.
Discussion
Our findings show that influenza seasonality is less well defined in the tropics than in temperate
regions. This has important implications for vaccination programmes in these countries.
High-quality influenza surveillance systems are needed in the tropics to enable
decisions about when to vaccinate.