Excerpts from WHO guidelines on booster doses for COVID-19 vaccination


Excerpts from WHO guidelines on
booster doses for COVID-19 vaccination

GENEVA, Switzerland: The World Health Organization, 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.1

Evidence of waning protection from primary vaccination series

The vast majority of current infections and COVID-19 cases are observed in unvaccinated people. If breakthroughs occur in vaccinated persons, in most cases events are less severe than those in unvaccinated persons. However, emerging data consistently show a decline in vaccine effectiveness against SARS-CoV2 infection and COVID-19 with time since vaccination, and more significant decline in older adults. This evidence is mostly based on observational studies that may be subject to confounding factors .

Developing a booster vaccination policy

At least 126 countries worldwide have already issued recommendations on booster or additional vaccination and more than 120 have started programmatic implementation. The majority of these countries are classified as high-income, or upper middle-income. No low-income country has yet introduced a booster vaccination programme. The most commonly prioritized target populations for booster doses are older adults, health workers and immunocompromised individuals (in immunocompromised individuals the booster dose is considered as an additional primary series vaccination dose by WHO). The degree of primary vaccination coverage in the eligible adult population varies. In several of these countries which are administering booster doses the coverage rates for complete primary vaccination are below 30%.


The focus of COVID-19 immunization efforts must remain on decreasing death and severe disease, and the protection of the health care system. Public health and social measures continue to be an essential component of the COVID-19 prevention strategy, especially in light of the Omicron variant. In the context of ongoing global vaccine supply constraints and inequities, broad-based administration of booster doses risks exacerbating vaccine access by driving up demand in countries with substantial vaccine coverage and diverting supply while priority populations in some countries, or in subnational settings, have not yet received a primary vaccination series.

Introducing booster doses should be firmly evidence-driven and targeted to the population groups at highest risk of serious disease and those necessary to protect the health system. To date, the evidence indicates a minimal to modest reduction of vaccine protection against severe disease over the 6 months after the primary series. Waning of effectiveness against all clinical disease and infection is more pronounced. Duration of protection against the Omicron variant may be altered and is under active investigation. Evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in high-risk populations, calls for the development of vaccination strategies optimized for prevention of severe disease, including the targeted use of booster vaccination.

More data will be needed to understand the potential impact of booster vaccination on the duration of protection against severe disease, but also against mild disease, infection, and transmission, particularly in the context of emerging variants. Over time, as vaccination programmes effectively protect populations from severe disease and death, the protection against milder disease and the reduction of transmission become important additional considerations.

SAGE has deliberated on the evidence for booster doses and the optimization of vaccination programmes during its Extraordinary SAGE meeting on 7 December 2021, which is reflected in this interim statement. SAGE will further discuss policies to optimize the use of vaccines including the consideration of booster vaccination at its forthcoming meeting on 19 January 2022. (PR)

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