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 studies show that the vaccines authorized in the United States protect very well against the worst effects of the disease, with some exceptions in the elderly and people with weakened immune systems.
But while the efficacy of the vaccines against serious illness and hospitalizations has remained largely stable even during 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 decreased.
Public health experts say the decline doesn’t mean vaccines don’t work.
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 approved by the Food and Drug Administration by 2020. But the importance of these drops in effectiveness — and whether they suggest that all adults should be eligible for a booster shot — is still up for debate.
A study in England examined the effectiveness of the vaccines against the delta variant over time. It was found that the Pfizer-BioNTech vaccine is about 90% effective in preventing symptomatic infection two weeks after the second dose, but drops to 70% after five months.
The same study found that the protection of the Moderna vaccine also decreases over time.
A study in the US and another in Canada looked at the effectiveness of the vaccines in preventing delta infection, whether symptomatic or not. While they found varying levels of decline, both studies found that the vaccines’ protection declined over time.
But both the UK and Canadian studies showed that the Pfizer-BioNTech and Moderna vaccines are still highly effective in preventing hospitalization even after several months.
Each of the three studies showed a different rate of decline in vaccine effectiveness, which can vary across studies, depending on factors such as the location, the study methods, and any behavioral differences between those vaccinated and unvaccinated. While one of the studies has been published, the other two have not yet been peer-reviewed. Still, experts say the research shows trends overall.
“The primary purpose of the COVID vaccine is to prevent serious illness and death, and they are still doing it well,” said Melissa Higdon, a faculty member at the Johns Hopkins Bloomberg School of Public Health who is leading a project to co-operate with research. on the performance of COVID vaccines.
But the decrease in protection against infection will have an effect, she said.
“With real declines in vaccine effectiveness, we’re probably going to see more cases overall,” Higdon said.
Data collected by the Centers for Disease Control and Prevention show similar trends for the mRNA vaccines, and they also suggest that the single-dose Johnson & Johnson vaccine is less effective against serious consequences and infection than Pfizer or Moderna.
These results helped shape current recommendations for boosters in the US: Of the Pfizer and Moderna recipients, over-65s and high-risk adults are eligible six months after their second injection. Any adult immunized with J&J can receive a booster after two months.
Pfizer and BioNTech last week asked the FDA to approve boosters for all adults. But experts are divided on whether booster shots are necessary for those beyond the most vulnerable.
There is greater agreement among experts about the need to provide additional protection for adults over 65. The observed decline in vaccine effectiveness for this age group may have greater implications, as the elderly are at higher risk of hospitalization.
“For those over 65, getting a booster to cover your base helps to make sure you have extra, extra protection because the impacts are greater,” said Eli Rosenberg, deputy director for science at the Office of Public Health from the New York State Department of Health, which has studied the effectiveness of COVID vaccines.
Seniors are also now most likely to be affected by declining vaccine immunity, as they were among the first to be vaccinated in the US. About 71% of people 65 and older – about 36 million people – completed their first vaccination course more than six months ago. So far, about 31% have received a booster injection.
Another 69 million people in the US who are under the age of 65, more than a quarter of that age group, are also past that six-month period. Not everyone is eligible for booster shots, although the federal government may soon decide to make anyone 18 and older eligible for the Pfizer booster.
Other countries, including Israel and Canada, have allowed booster shots for all adults. Early data from Israel shows that booster shots are effective in protecting against infection and hospitalization, at least in the short term.
But experts fear a national focus on boosters will detract from what should be the country’s main goal.
“It’s easy with all the discussion about boosters to lose that really important message that the vaccines still work,” Rosenberg said. “Going from an unvaccinated to a vaccinated person is still the critical step.”