The data suggest that the booster will enhance protective antibody responses. This would likely improve the transmission-blocking component of vaccination, potentially increasing the window of time that an individual will be resistant to infection and subsequent transmission. However, for protection from severe disease, the current data suggest that the booster vaccine is likely not necessary and that we should be prioritizing vaccination of unvaccinated individuals over administration of third shots. ​However, certain individuals would likely benefit from a third dose, including the elderly, immunocompromised, and those at high risk for SARS-CoV-2 infection.

There is good data from the U.S. that shows the two-dose Moderna series continues to show strong protection against severe disease with the delta variant (93% protection against hospitalizations in a Centers for Disease Control and Prevention analysis performed in 18 states). The Moderna vaccine was also associated with a two-fold risk reduction in developing mild breakthrough cases in the Mayo Clinic compared to the Pfizer vaccine. The Moderna booster has only been recommended for selected groups (immunocompromised, 65 and older, those younger with medical conditions, and those in occupations with frequent exposure) and, given the continuing strong protection of the two-dose series in U.S. data, I think individuals should only consider this booster if immunocompromised or over 65, especially with other medical conditions.

Those who received the Moderna vaccine should be reassured that, regardless of whether or not they choose to receive a booster, as of now protection from severe disease, hospitalization, and death remains excellent. The big advantage of a booster dose in this group is that it will provide better, higher-level immunity that will not only provide added protection from severe disease, but also likely from being infected at all with the delta variant. In Israel, which has the most data on the real-world impact of third doses, the likelihood of becoming infected plummeted about two weeks after receiving a third dose. Many people, and I include myself in this group, would love to reduce their risk of getting infected as much as possible. Having a group of people who are at dramatically lower risk of becoming infected at all may also help act as a metaphorical firebreak, reducing the amount of virus in communities and reducing everyone’s risk of becoming infected.

We know that Moderna and Pfizer vaccines both continue to protect vaccinated individuals against severe disease, hospitalization, and death. Vaccine-generated immunity does wane with time, however, so the Food and Drug Administration advisers have recommended a booster strategy designed to extend protection for the following groups:

• individuals 65 years of age and older;
• individuals 18 through 64 years of age at high risk of severe COVID-19; and
• individuals 18 through 64 years of age whose frequent institutional or occupational exposure to SARS-CoV-2 puts them at high risk of serious complications of COVID-19 including severe COVID-19.

This puts the Moderna booster on equal footing with the Pfizer booster. The committee evaluated the data available regarding waning immunity, including some data from Israel that suggests a booster can help reduce severe disease in elderly individuals. There remains some debate on the overall need for boosters. However, there is no doubt that the titers of neutralizing antibodies (a key measure of vaccine responses and potential indicator of protection) rise substantially following a third dose of either of the mRNA vaccines.

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