Studies show efficacy of vaccines wanes, fueling booster shots debate

As tens of millions of eligible people in the United States consider signing up for a COVID-19 booster shot, a growing body of early global research shows that the vaccines authorized in the United States remain highly protective against the disease’s worst outcomes over

time, with some exceptions among older people and those with weakened immune systems.

But although the vaccines’ effectiveness against severe disease and hospitalization has mostly held steady, even through the summer surge of the highly transmissible delta variant, a number of published studies show that their protection against infection, with or without symptoms, has fallen.

Public health experts say the decline doesn’t mean vaccines aren’t working.

In fact, many studies show that the vaccines remain more than 50% effective at preventing infection, the level that all COVID vaccines had to meet or exceed to be authorized by the Food and Drug Administration back in 2020. But the significance of these declines in effectiveness — and whether they suggest all adults should be eligible for a booster shot — is still up for debate.

A study in England examined the vaccines’ effectiveness against the delta variant over time. It found that the Pfizer-BioNTech vaccine is about 90% effective at preventing symptomatic infection two weeks after the second dose but drops to 70% effective after five months.

The same study found that the Moderna vaccine’s protection also drops over time.

A study in the U.S. and another in Canada looked at the vaccines’ effectiveness at preventing any infection from delta, symptomatic or not. Although they found different levels of decline, both studies

found that the vaccines’ protection dropped over time.

But both the English and Canadian studies found that even after several months, the Pfizer-BioNTech and Moderna vaccines remain highly effective at preventing hospitalization.

Pfizer and BioNTech have asked

the FDA to approve boosters for all adults. But experts are divided

over whether booster shots are necessary for those beyond the most vulnerable.

There has been more agreement among experts about the need to offer extra protection to adults older than 65. The declines observed in vaccine effectiveness for this age group may have greater repercussions, since older people face a higher risk of hospitalization.

Some experts worry that a national focus on boosters will detract from what should be the country’s most important goal.

“It’s easy with all the discussion about boosters to lose that really

important message that the vaccines are still working,” said Eli Rosenberg, deputy director for sci

ence in the Office of Public Health at the New York State Department of Health.