The World Health Organization (WHO), with support of the Strategic Advisory Group of Experts (SAGE) on Immunization and its COVID-19 Vaccines Working Group, is reviewing the emerging evidence on the need for and timing of an additional vaccine dose (booster dose 1) for the currently available COVID-19 vaccines which have received Emergency Use Listing (EUL).
SAGE is continuously reviewing the literature and has reached out to vaccine manufacturers, the research community and Member States to obtain the most complete and recent data on the issue. .
Factors to consider:
- Waning immunity
- Vaccine effectiveness
- Global vaccine supply and global and national equity
The duration of vaccine-induced protection is likely to depend on many variables, such as the vaccine product, the primary vaccination schedule, the age and/or underlying medical conditions of the vaccine, risk of exposure and circulation of specific variants. The decision to recommend a booster dose is complex and requires, beyond clinical and epidemiological data, a consideration of national strategic and programmatic aspects, and importantly an assessment of the prioritization of globally limited vaccine supply. In this context, prioritization should be given to the prevention of severe disease.
Data needs can be grouped into the following categories:
Assessing the need for booster doses:
- Epidemiology and burden of disease
- Epidemiology of breakthrough cases and disease severity by age, co-morbidity and risk groups, exposure, type of vaccine, time since vaccination and in the context of variants of concern (VOCs).
- Vaccine-specific data
- Efficacy, effectiveness, duration of protection of vaccines in the context of SARS-CoV-2 wild-type and VOCs from observational studies and randomized controlled trials.
- Supplementary evidence from immunological studies assessing neutralizing antibodies over time, as well as biomarkers of cellular immunity when possible.
Assessing the performance of booster doses:
- Immunogenicity, efficacy, effectiveness and duration of protection of original and variant-adapted vaccine booster doses in the context of SARS-CoV-2 wild-type and VOCs.
- Safety and reactogenicity.
Additional considerations:
- Optimal timing of the booster dose, consideration of homologous versus heterologous boosters, possibility for dose-sparing for booster doses, booster needs in previously infected individuals, specification and prioritization of high-risk populations, programmatic feasibility and sustainability, promotion of global equity.
In the context of ongoing global vaccine supply constraints, administration of booster doses will exacerbate inequities by driving up demand and consuming scarce supply while priority populations in some countries, or subnational settings, have not yet received a primary vaccination series. The focus for the time being remains on increasing global vaccination coverage with the primary series (either one or two doses for current EUL vaccines).
Introducing booster doses should be firmly evidence-driven and targeted to the population groups in greatest need. The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in the general population or in high risk populations, or due to a circulating VOC. To date, the evidence remains limited and inconclusive on any widespread need for booster doses following a primary vaccination series. WHO is carefully monitoring the situation and will continue to work closely with countries to obtain the data required for policy recommendations.
