Infection study shows steep decline in vaccine effectiveness over eight months

November 9, 2021
Infection study shows steep decline in vaccine effectiveness over eight months

A study by the Public Health Institutes (PHI) has detected a sharp decline in the protection from infection conferred by three major COVID-19 vaccines among US veterans. At the same time, the vaccines’ protection from death remained strong across the study period from February 2021 to October 2021. PHI researchers hypothesise the Delta variant of the coronavirus may be responsible for this drop in vaccine protection from infection instead of the vaccine itself becoming less protective over time.

In the PHI study, which looked at health records from around 780,000 veterans, vaccine effectiveness (VE) of the three COVID-19 vaccines approved for use in the US were compared: Moderna, Pfizer-BioNTech (Pfizer) and Johnson & Johnson (Janssen). VE was measured by two metrics: protection from COVID-19 infection (VE-I) and protection from death (VE-D).

Vaccine protection against COVID-19 infection dropped from 87.9% to 48.1% over the eight-month study period. The most dramatic decline in VE-I was seen in the Janssen vaccine, dropping from 86.4% to 13.1%; Pfizer’s dropped from 86.9% to 43.3%; while Moderna’s dropped from 89.2% to 58%.

However, the study did not detect a significant decline in vaccine protection against death. Between July 2021 and October 2021, the vaccines were still 81.7% effective at preventing death from COVID-19 in those under the age of 65 and 71.6% effective in those over the age of 65 – the Pfizer vaccine conferred the greatest protection against death (84.3%) for those under the age of 65, but in those over the age of 65 the Moderna vaccine was most effective at preventing death (75.5%).

Researchers at PHI said the drop in vaccine protection against infection across all age groups is likely due to the Delta variant. Findings from another recent vaccine effectiveness study from the Centers for Disease Control and Prevention (CDC) and the New York State Department of Health also detected declines in vaccine protection against infection, when the Delta variant rapidly spread across the US.

The researchers stress that vaccination provided protection against death in infected persons, as observed for the Moderna, Pfizer, and Janssen vaccines during the Delta surge (the benefit was greater for Moderna and Pfizer compared to Janssen vaccines).

“Our findings support the conclusion that COVID-19 vaccines remain the most important tool to prevent infection and death,” the researchers said.

Dr. Barbara Cohn of PHI argued that the study points to the need for additional measures, beyond merely vaccination, in reducing the impact of COVID-19 over the coming months, “… given the declines in vaccine protection and the dominance of the more infective Delta variant, we urge swift action to promote primary vaccination, boosters and to also encourage masking, social distancing and other layers of protection against infection.”

Category: Education, Features

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