Based on the start of the primary influenza activity period, countries are assigned to two categories:
- countries where the primary influenza activity starts after October, and,
- countries where the primary influenza activity starts after April.
In order to be most effective in countries with multiple peaks and residual year-round activity, WHO recommends that the seasonal influenza vaccine should be given prior to the start of the primary period of increased influenza activity.
Countries are encouraged to analyse local surveillance information to assess their seasonality pattern at both national and sub-national level as appropriate to make evidence-based decisions on timing of vaccination campaigns.