Pfizer, Moderna Vaccines Less Effective Against Delta In Elderly And Protection Wanes Rapidly Four Months After Second Covid Shot, Study Finds

Topline

Pfizer and Moderna’s Covid-19 vaccines are much less effective at preventing infection of senior citizens with the delta coronavirus variant and protection fades rapidly four months after the final jab, new research published in JAMA Network Open has found, as experts race to understand how effective the shots are against the fast-spreading, potentially vaccine-resistant omicron variant.

Protection against infection fades rapidly after four months for Pfizer and Moderna shots in elderly … [+] men, a study found.

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Key Facts

Before the delta variant, the mRNA vaccines were 95% effective at preventing coronavirus infection in the first month after the final shot, falling to 88% by the third month, according to an analysis of health records from over 14,000 Covid-positive male veterans over the age of 65.

Through July to September, a period of high delta transmission in the U.S., protection against infection had fallen to 62% in the first month after the second shot and 58% by the third month, a similar pattern from the pre-delta period.

After the third month—which the researchers could only during times of high delta transmission—vaccine effectiveness decreased at an accelerating rate, falling to 38% during month four to a low of around 20% in months five through seven.

The decrease could be due to both increased transmission of the delta variant and vaccine immunity waning over time, the researchers said.

The results are in line with other studies documenting decreasing vaccine effectiveness over time which accelerates after month four following full vaccination, the researchers said, noting that the study’s focus on older male veterans should be considered if trying to generalize the findings.

Key Background

Experts and officials have long warned the immunity provided by vaccines may diminish over time and booster shots—an additional dose of vaccine following the initial course—are frequently deployed in medicine as a way of countering this. One day, it’s possible the definition of “fully vaccinated” will be expanded to encompass a third coronavirus vaccine shot, a regimen common for other vaccines. While preventing infection is important for controlling the spread of disease, it is not the only purpose of vaccination. The shots are still effective at preventing serious illness, hospitalization and death from Covid-19 and experts believe there is strong evidence the vaccines still do this.

What We Don’t Know

Early evidence suggests the shots are much less effective at preventing infection with the omicron variant, though still offer strong protection against severe disease. Research into omicron is in its early stages and more information will emerge over time.