With more evidence, integrating infection and vaccination-induced immunity into vaccination strategies and/or schedules may provide gains through simplified and/or more effective immunization schedules in countries or communities that have already experienced high levels of community transmission. However, basing national vaccination policies on seroprevalence rates poses several challenges. Seroprevalence rates observed in population-based studies may not be representative of the entire population or certain subpopulations and age groups, and may also differ by factors such as urban versus rural settings. Moreover, hybrid immunity1, although superior to infection- or vaccine induced immunity alone, depends on a number of factors in complex and interrelated ways.
Meanwhile, WHO reiterates the need for accruing consistent population representative data over time to further improve our understanding of waning of seroprevalence and hybrid immunity. In particular, seroprevalence studies are needed in specific at-risk populations such as older age groups which may inform vaccine priorities and targeted efforts in the future. An improved understanding of hybrid immunity will contribute to evidence-based decisions on the need for additional Covid-19 vaccine doses to populations. When more evidence is available, advice on if and how hybrid immunity should be considered in national vaccination policies will be updated.
Note :
1- Hybrid immunity is defined as the immune protection in individuals who have had one or more doses of a Covid-19 vaccine and experienced at least one SARS-CoV-2 infection before or after the initiation of vaccination.