As the Delta variant became the dominant strain of coronavirus across the United States, all three COVID-19 vaccines available to Americans lost some of their protective power, with vaccine efficacy among a large group of veterans dropping between 35% and 85%, according to a new study.
Researchers who scoured the records of nearly 800,000 U.S. veterans found that in early March, just as the Delta variant was gaining a toehold across American communities, the three vaccines were roughly equal in their ability to prevent infections.
But over the next six months, that changed dramatically.
By the end of September, Moderna’s two-dose COVID-19 vaccine, measured as 89% effective in March, was only 58% effective.
The effectiveness of shots made by Pfizer and BioNTech vaccine, which also employed two doses, fell from 87% to 45% in the same period.
And most strikingly, the protective power of Johnson & Johnson’s single-dose vaccine plunged from 86% to just 13% over those six months.
The findings were published Thursday in the journal Science.
The three vaccines held up better in their ability to prevent COVID-19 deaths, but by July — as the Delta variant began to drive a three-month surge of infections and deaths — the shots’ effectiveness on that score also revealed wide gaps.