The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease with the onset of breakthrough cases and reinfections, or due to the circulation of a variant of concern of SARS-COV-2 capable of evading the immune response induced by the current vaccine schedules.
According to the WHO, introducing booster doses (namely, a 3rd dose to people who have been vaccinated with 2 doses of the Pfizer-BioNTech, AstraZeneca, or Moderna vaccines, or a 2nd dose to people who have been vaccinated with 1 dose of the Janssen vaccine) should be firmly evidence-driven and targeted to the population groups in greatest need. To date (September 2021), the evidence remains limited and inconclusive on any widespread need for booster doses following a primary vaccination series [^1].
It is important to distinguish between booster doses and a limited fraction of the population who may need an additional dose for primary response. Emerging data suggests that immunocompromised people (e.g. those with solid organ transplants, hemodialysis) should receive one more dose in their primary vaccination series if their immune system has not responded sufficiently to the primary series, as can be the case with most other vaccines in these situations.
In the context of ongoing global vaccine supply constraints, administration of booster doses in highly vaccinated populations 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. At present, the global priority remains to provide a full vaccination course, which is higly effective in preventing serious disease, hospitalizations and death from currently circulating variants of the virus, to those who have not received any COVID-19 vaccine and to complete the vaccination schedule for those who have not done so yet.
WHO is carefully monitoring the situation and will continue to work closely with countries to obtain the data required to update or modify policy recommendations.
- Evidence remains limited and inconclusive on any widespread need for booster doses.
- Immunocompromised people may need a third dose to achieve the same level of immunity as others in the general population.
- The limited supply of vaccines will save the most lives if made available to people who are at appreciable risk of serious disease and have not yet received any vaccine.