In a joint statement from the Centers for Disease Control and Prevention (CDC), FDA, and National Institutes of Health (NIH), the US Government agencies announced a plan to recommend Covid-19 booster vaccine doses for all residents eight months after full vaccination with either Pfizer/BioNTech or Pfizer vaccines, beginning in mid-September. This is a switch from the previous guidance of the agencies to not recommend booster shots in the near future, but the highly transmissible delta variant and rising Covid-19 case numbers in the US led to the change in strategy to combat Covid-19 domestically.

Initial data from Pfizer/BioNTech, Moderna, AstraZeneca, and Novavax showed that a third dose of a Covid-19 vaccine, given six to eight months after the second dose, can significantly boost the protection against the ancestral SARS-CoV-2 strain, and potentially also against virus variants. Thus, the guidance to recommend a third vaccine dose (or second dose in the case of Johnson & Johnson’s single-dose vaccine, when a booster is advised for it) is an important decision. Furthermore, a third dose could also broaden the protection and potentially create immunity against emerging SARS-CoV-2 variants. However, this decision will further increase the gap in protection against the novel coronavirus and its variants between high and low-income countries. Globally, less than 25% of people have been fully vaccinated, while only 85 million Covid-19 vaccine doses have been administered in all of Africa.

Also, countries such as Germany, France, the UK, and Israel are planning to recommend a third dose for immunocompromised people before the colder season in the Northern Hemisphere begins. A strategy that could prioritize high-risk people in highly vaccinated countries, while making sure enough vaccine doses reach regions in which only a small percentage of the population is fully vaccinated, might be a compromise to prevent the gap in protection from broadening further. Eventually, once Covid-19 vaccine supply is a non-issue, a third dose, like for other vaccines, especially childhood vaccines, will be a good strategy to create long-term protection against SARS-CoV-2.

Additionally, should variants emerge that escape the immune response of people who have received three doses of the vaccine, variant-specific boosters that are currently in development will again be needed, especially for those at high risk, like the elderly, particularly in long-term care facilities, the immunocompromised, and healthcare workers. These will be the groups most affected by new SARS-CoV-2 variants, which will continue to emerge until most of the world’s population is vaccinated.