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Policy & Global Development

COVID-19 Vaccines: What Are the Global Health Consequences of a Third Dose?

Series | COVID-19 and response strategy #39


[This document is a one of a series of discussion notes addressing fundamental questions about the COVID-19 crisis and response strategies. These documents are based on the best scientific information available and may be updated as new information comes to light.]

Written by Clara Marín, Adelaida Sarukhan and Marta Rodó (ISGlobal), this document is partially based on a Statement on the third dose of COVID-19 vaccines (16/09/2021) by the Multidisciplinary Collaborative Group for the Scientific Monitoring of COVID-19.


Most countries with high COVID-19 vaccination rates have embarked on a new debate about the need for a third dose. But, who should receive a third dose? Does a third dose make sense for the general population? Is it ethical to administer third doses at a time when a majority of the population in many countries has yet to receive the initial regimen? Is an extra dose essential to control the pandemic?

There is no direct evidence that a booster dose increases protection against the virus, as the European Medicines Agency (EMA) specified in a recent statement that supported the consideration of a possible third dose for the general population. Under the precautionary principle, this measure could have short-term benefits in vaccinated populations that are at low risk of adverse effects

However, these recommendations do not incorporate a global health vision. Introducing a third dose for populations that are already vaccinated will always be associated with exceptional localised benefits. But these benefits must be viewed in the context of a global battle against the virus in which no one is safe until everyone is safe.

The most effective strategy for controlling the pandemic and dealing with new viral variants is to increase vaccine coverage globally, rather than providing booster shots to the vaccinated population, with the exceptions of some groups of immunocompromised patients and people over 65 years of age.