The efficacy of third doses of the Pfizer and Moderna mRNA vaccines wanes substantially by the fourth month after administration, a new study by the US Centers of Disease Control and Prevention has said.
Though it is now well documented that vaccine efficacy goes down after two doses, relatively little has been published on the duration of protection after a booster.
The new study was based on more than 241,204 visits to an emergency care clinic and 93,408 hospitalisations among adults with Covid-19-like illness between 26 August, 2021 and 22 January, 2022.
Vaccine efficacy was estimated by comparing the odds of a positive Covid test between vaccinated and unvaccinated patients and using statistical methods to control for calendar week and geographic area, The study also adjusted for age, the level of local transmission, and patient characteristics like co-morbidities.