According to a new study from the US Centers for Disease Control and Prevention (CDC), the efficacy of third doses of the Pfizer and Moderna mRNA vaccines declines significantly by the fourth month after administration.
Though it is now well established that vaccine efficacy declines after two doses, little is known about the duration of protection after a booster.
The new study was based on over 241,204 visits to the emergency department or an urgent care clinic, as well as 93,408 more serious hospitalizations among adults with Covid-19–like illness from August 26, 2021, to January 22, 2022.
Vaccine efficacy was calculated by comparing the odds of a positive Covid test between vaccinated and unvaccinated patients and using statistical methods to control for a calendar week, geographic area, and age, as well as patient characteristics such as chronic conditions.
During the Omicron-predominant period, vaccine efficacy against Covid-associated emergency department or urgent care visits was 87 percent after the third dose but dropped to 66 percent by the fourth month.
Vaccine efficacy against hospitalization was 91 percent in the first two months but dropped to 78 percent after a third dose in the fourth month.
President Joe Biden‘s top medical advisor, Anthony Fauci, said at a White House Covid briefing on Wednesday that fourth doses would be more likely needed for subsets of people who mount weaker immune responses, such as the elderly and immunocompromised.