The World Health Organization (WHO), with the support of the Strategic Advisory Group of Experts (SAGE) on Immunization and its COVID-19 Vaccines Working Group, continues to review the emerging evidence on the need for and timing of a booster dose for the currently available COVID-19 vaccines which have received Emergency Use Listing (EUL).
This statement reflects the current understanding of vaccine performance and supply as presented to SAGE on 7 December 2021. It summarizes and contextualizes current evidence on booster vaccination. In recent weeks the SARS-CoV2 Omicron variant has emerged. Data are currently insufficient to assess the impact of this new variant of concern on vaccine effectiveness, in particular against severe disease. The statements and conclusions in this document will therefore be updated as data become available.
The following definitions and terminology are used by WHO throughout its policy recommendations on COVID-19 vaccination. This note focuses only on booster doses.
Booster doses are administered to a vaccinated population that has completed a primary vaccination series (currently one or two doses of EUL COVID-19 vaccine depending on the product) when, with time, the immunity and clinical protection has fallen below a rate deemed sufficient in that population. The objective of a booster dose is to restore vaccine effectiveness from that deemed no longer sufficient.
Additional doses of a vaccine may be needed as part of an extended primary series for target populations where the immune response rate following the standard primary series is deemed insufficient. The objective of an additional dose in the primary series is to enhance the immune response to establish a sufficient level of effectiveness against disease. In particular, immunocompromised individuals often fail to mount a protective immune response after a standard primary series, but also older adults may respond poorly to a standard primary series with some vaccines.
Vaccine booster dose policy decisions should be based on evidence of individual and public health benefit and obligations to secure global equity in vaccine access as a means to minimize health impacts and transmission, and thereby reduce the risk of variants and prolongation of the pandemic. While vaccine supply is growing, it is not evenly distributed. Lower income countries have had far less access, and face unpredictable and irregular supply. Within countries, equity considerations support improving coverage of the primary vaccination series in high risk populations as the top priority use of vaccine doses.
The WHO roadmap on the prioritization of vaccine use in situations of limited supply (hereafter: the Roadmap), as well as the Strategy to Achieve Global COVID-19 Vaccination by mid-2022, define a hierarchy of public health goals in accordance with progressively increasing control of the COVID-19 pandemic. Among those, the primary global goal for the acute phase of the pandemic is to reduce deaths and severe disease due to COVID-19 and to protect the health system. The level of population vaccination coverage needed to achieve this goal may differ between countries.
Subsequent public health goals include the reduction of COVID-19 disease burden and of viral transmission, to restore social and economic life, as described in WHO’s Strategy to Achieve Global COVID-19 Vaccination by mid-2022. These goals and vaccine use cases should be pursued only when priority risk groups have full access to vaccines in order to achieve the primary objective of substantial reduction in severe disease and mortality.