Efficacy of Pfizer and Moderna vaccines wane after third dose: Study

A new study by the US Centers of Disease Control and Prevention (CDC) has said that the efficacy of third doses of the Pfizer and Moderna mRNA vaccines wanes substantially by the fourth month after administration.

CDC examined data on 93,000 hospitalisations and 241,000 emergency department and urgent care visits across 10 states during the Delta and Omicron waves. In the study, about 10 per cent of people were boosted and over 50 per cent of people hospitalised were over 65 years old. 

During Omicron, vaccine effectiveness against hospitalisation was 91 per cent during the first two months after a third dose and remained high, at 78 per cent, four or more months after a third dose.

Study participants were recruited from the companies' late-stage COVID-19 vaccine study, and those who had received the second dose of the company's COVID-19 vaccine at least six months before entering the co-administration study.

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The study reveals that a third dose of mRNA vaccine continues to offer high levels of protection against severe disease, even months after administration, underscoring the importance of staying up to date when eligible after receiving a primary series.

"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.

Pfizer Inc Chief Executive Albert Bourla said that an annual COVID-19 vaccine would be preferable to more frequent booster shots in fighting the coronavirus pandemic.

Pfizer/BioNtech's COVID-19 vaccine has shown to be effective against severe disease and death caused by the heavily-mutated Omicron variant but less effective in preventing transmission.

With cases soaring, some countries have expanded COVID-19 vaccine booster programs or shortened the gap between shots as governments scramble to shore up protection.

(With inputs from agencies)