The efficacy of third doses of the Pfizer and Moderna mRNA vaccines falls substantially by the fourth month after administration, according to the latest study by the US Centers of Disease Control and Prevention (CDC).

The new study was based on more than 241,204 visits to the emergency department or an urgent care clinic, and 93,408 hospitalizations, which are more serious, among adults with Covid-19 like symptoms during August 26, 2021- January 22, 2022.

Vaccine efficacy was estimated by comparing the odds of a positive Covid test between vaccinated and unvaccinated patients.

It was found that during the period dominated by the Omicron variant of COVID-19, vaccine efficacy against Covid-associated emergency department or urgent care visits was 87 percent during the two months after a third dose, but fell to 66 percent by the fourth month.

Vaccine efficacy against hospitalization was 91 percent in the first two months, but fell 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.

Speaking at a White House Covid briefing, President Joe Biden's top medical advisor Anthony Fauci said it was likely that fourth doses would more likely be needed for subsets of people who mount weaker immune responses, such as the elderly and immunocompromised.

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