Adapting delivery strategies to influenza seasonality

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Gaps in Vaccine Availability

WHO recommends the use of the most recent vaccine formulation available. Following strain selection and recommendation by WHO, influenza vaccine is formulated by manufacturers as Northern Hemisphere (NH) and Southern Hemisphere (SH) vaccine and is – depending on regulatory approval and delivery times accessible to countries around September (NH) or April (SH). Seasonality of influenza by region impacts the timing of vaccination.

  • In temperate regions, shortly after the respective hemisphere’s vaccine is available, vaccination is usually offered during a specific period before the start of the influenza season and can be offered to pregnant women through routine immunization during the remaining influenza season.
  • In tropical and subtropical regions where there are often no distinct seasonal peaks, information on the country’s influenza seasonality and virus characteristics should be used to choose either NH or SH vaccine [31, 32]. If influenza vaccination campaigns are used, they should be timed before the main peak of the influenza season. Where no data on national influenza seasonality are available, countries can use estimates from countries with similar seasonality. For a list of countries see publication Seasonal Influenza seasonality in low and middle income countries in the tropics and subtropics. In practice, where coverage during several peaks is not feasible, influenza vaccine campaigns are often timed prior to onset of the primary seasonal peak. In such cases, offering routine coverage with remaining vaccine over several months can help to improve coverage among pregnant women.
  • In some countries close to the equator, influenza virus circulates all year round, without distinct seasonal peaks, requiring additional considerations on vaccine composition, availability and programmatic issues. Offering vaccination through routine delivery services offers protection from circulating viruses over an extended period of time. The feasibility and ability of this delivery strategy would, however, need to be assessed on the basis of vaccine availability and local programmatic considerations.
    Vaccine production and regulatory approval processes typically lead to “availability gap months” of approximately three months per year for each SH or NH vaccine.


How should a country choose NH or SH vaccine in tropical and subtropical regions where there are often no distinct seasonal peaks:

A. Follow the lead of neighbouring countries.
B. Use both vaccines.
C. Consider information on influenza seasonality and virus characteristics .
D. Use either NH or SH vaccine.


Answer C is correct.