Washington: The effectiveness of third doses of the Pfizer and Moderna mRNA vaccines wanes considerably by the fourth month after administration, a new study by the US Centers of Disease Control and Prevention (CDC) revealed on Friday.
Though it's now well documented that vaccine's effectiveness decreases after two doses, comparably, little has been said on 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, and 93,408 hospitalizations, which are more severe, among adults with COVID-19 like illness during August 26, 2021- January 22, 2022, NDTV reported.
The efficacy of vaccine was estimated by comparing the odds of a positive COVID test between vaccinated and unvaccinated patients and making the use of statistical methods to control for calendar week, geographic area, while adjusting for age, the level of local transmission, and patient characteristics like comorbidities.
During the Omicron-predominant period, efficacy of the vaccine against Covid-related emergency department or urgent care visits was 87 percent within the two months after a third jab, but dropped to 66 percent by the fourth month.
Vaccine efficacy against hospitalization was 91 percent during the first two months, but dropped to 78 percent by the fourth month after a third dose.
"The finding that protection conferred by mRNA vaccines waned in the months after receipt of a third vaccine dose reinforces the importance of further consideration of additional doses to sustain or improve protection," the authors concluded.
Addressing a White House Covid briefing on Wednesday, Anthony Fauci, the US President Joe Biden's top medical advisor said it was likely that fourth doses would more likely be needed for people with weaker immune responses, such as the elderly and immunocompromised.
