Update to the WHO interim statement of the Strategic Advisory Group of Experts on Immunization regarding booster doses for COVID-19 vaccination


On 4 October 2021, the Strategic Advisory Group of Experts (SAGE) on Immunization and its COVID-19 Vaccines Working Group updated the interim statement on COVID-19 booster doses for the vaccines included in the Emergency Use Listing (EUL). 

They note that, at present, the main purpose of immunization against COVID-19 continues to be to protect against hospitalization, serious illness, and death. Booster doses may only be needed if there is evidence of insufficient protection against these disease outcomes over time. In a period of continued global vaccine supply shortage, equity considerations at the country, regional, and global levels remain an essential factor in assuring vaccination of highpriority groups in all countries, and indicate that the following factors should be considered:

  1. Waning immunity: Neither an immune correlate of protection nor of duration of protection has been established to date. Although there may be a loss of protection against infections by SARS-CoV-2, protection against severe disease is more durably retained due to anamnestic humoral and cell-mediated immunity.
  2. Vaccine effectiveness: Emerging data from observational studies consistently show a decline in vaccine effectiveness against infection and milder forms of COVID-19 over time. With respect to duration of protection against disease requiring hospitalization, current data show an overall continued high level of effectiveness, although data vary according to type of vaccine, age group, and target population. The vast majority of current infections are observed in unvaccinated populations, and if breakthrough infections occur in vaccinated persons, they are in most cases less severe than those seen in unvaccinated persons.
  3. Global vaccine supply and global and national equity: National vaccination program policy decisions to add a booster dose should take into account the strength of evidence regarding the need for these doses, their safety and effectiveness, as well as the global availability of vaccines. Prioritizing booster doses over speed and breadth in the initial dose coverage may also damage the prospects for global mitigation of the pandemic, with severe implications for the health and social and economic well-being of people globally. 

In assessing the need for booster doses, the following should be taken into account:

  • Epidemiology and burden of disease: Epidemiology of breakthrough cases, by disease severity, age, comorbidity, risk group, exposure, type of vaccine, time since vaccination, and context with regard to VoCs.
  • Vaccine-specific data: Efficacy, effectiveness, duration of protection of vaccines, in the context of circulating VoCs, based on observational studies and, if possible, randomized controlled trials. Supplementary evidence from immunological studies assessing binding and neutralizing antibodies over time, as well as biomarkers of cellular and durable humoral immunity, when possible, should also be considered.
  • Assessing the performance of booster doses: While preliminary data on effectiveness of booster vaccinations have been obtained only for the Pfizer-BioNTech vaccine (1), additional data on 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 would be helpful. Safety and reactogenicity of booster vaccination, including heterologous boosting, also need to be studied.
  • Additional considerations: Optimal timing of the booster dose; homologous versus heterologous boosters; possibility of dose-sparing for booster doses; booster needs in previously infected individuals; specification and prioritization of high-risk populations; programmatic feasibility and sustainability; community perception and demand, as well as equity considerations.


SAGE will deliberate on the evidence for a booster dose during an upcoming extraordinary meeting of SAGE in November 2021.


Source: https://www.who.int/news/item/04-10-2021-interim-statement-on-booster-doses-for-covid-19-vaccination

(1) Bar-On YM, Goldberg Y, Mandel M, Bodenheimer O, Freedman L, Kalkstein N et al. Protection of BNT162b2 Vaccine Booster against Covid-19 in Israel. N Engl J Med. 2021. doi: 10.1056/NEJMoa2114255. 

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