Report: New Study Shows Drastic Decline In COVID Vaccine Effectiveness Over Time

Published|Updated

All three COVID-19 vaccines available to Americans became less effective as the Delta variant became dominant in the U.S., with vaccine efficacy among a large group of veterans dropping between 35% and 85%, according to a new study published Thursday.

Researchers who collected the records of nearly 800,000 U.S. veterans found that as the Delta variant was emerging across American communities in March, the three vaccines were roughly equal in their ability to prevent infections.

Over the next six months, this wouldn't be the case with Moderna’s two-dose COVID-19 vaccine — measured as 89% effective in March — was only 58% effective by the end of September, the study shows.

The study states the effectiveness of Pfizer's two-dose vaccine fell from 87% to 45% in the same time frame.

Figure 1: Study shows dramatic decline in effectiveness of all three COVID-19 vaccines over time

Vaccine effectiveness presented as (1 – hazard ratio × 100) and 95% confidence intervals. Effectiveness for each month was estimated from contrasts using product terms for vaccination status by time to most recent RT-PCR assay. Courtesy of "SARS-CoV-2 vaccine protection and deaths among US veterans during 2021."

As the Delta variant became the predominant variant in the U.S., the efficacy of Johnson & Johnson’s single-dose vaccine plunged from 86% to just 13% over those six months, the study shows.

The study — which was conducted by a team from the Public Health Institute in Oakland, the Veterans Affairs Medical Center in San Francisco, and the University of Texas Health Science Center — concluded that the Delta variant was likely the factor to look to in terms of reduced efficiency.

"The Janssen vaccine showed the greatest decline in . Breakthrough infections were not benign as vaccinated persons and who were subsequently infected had a higher risk of death compared to vaccinated persons who remained infection-free," the study reads. "Importantly, vaccination still provided protection against death in infected persons, and this benefit was observed for the Moderna, Pfizer-BioNTech, and Janssen vaccines during the Delta surge, although the benefit was greater for Moderna and Pfizer-BioNTech compared to Janssen vaccines. Our findings support the conclusion that COVID-19 vaccines remain the most important tool to prevent infection and death."

The findings were published Thursday in the journal Science, and the study can be read in full here.

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