Vaccine Effectiveness Against Infection May Wane, C.D.C. Studies Find

The Centers for Disease Control and Prevention released three studies on Wednesday that federal officials said provided evidence that booster shots of the Pfizer-BioNTech and Moderna coronavirus vaccines would be needed in the coming months.

But some experts said the new research did not back up the decision to recommend booster shots for all Americans.

Taken together, the studies show that although the vaccines remain highly effective against hospitalizations and deaths, the bulwark they provide against infection with the virus has weakened in the past few months.

The finding accords with early data from seven states, gathered this week by The New York Times, suggesting a rise in breakthrough infections and a smaller increase in hospitalizations among the vaccinated as the Delta variant spread in July.

The decline in effectiveness against infection may result from waning vaccine immunity, a lapse in precautions like wearing masks or the rise of the highly contagious Delta variant, experts said — or a combination of all three.

“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death,” Dr. Vivek Murthy, the surgeon general, said at a White House news briefing on Wednesday.

Citing the data, federal health officials outlined a plan for Americans who received the two vaccines to get booster shots eight months after receiving their second doses, starting Sept. 20.

People who received the Johnson & Johnson vaccine may also require additional doses. But that vaccine was not rolled out until March 2021, and a plan to provide boosters will be made after reviewing new data expected over the next few weeks, officials said.

Some scientists were skeptical of the administration’s new initiative.

“These data support giving additional doses of vaccine to highly immunocompromised persons and nursing home residents, not to the general public,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center and a former adviser on the pandemic to the administration.

Boosters would only be warranted if the vaccines were failing to prevent hospitalizations with Covid-19, she said.

“Feeling sick like a dog and laid up in bed, but not in the hospital with severe Covid, is not a good enough reason” for a campaign of booster shots, Dr. Gounder said. “We’ll be better protected by vaccinating the unvaccinated here and around the world.”

It’s unclear whether a third dose would help people who did not produce a robust immune response to the first two doses, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

And the recommendation for boosters may also end up undermining confidence in the vaccines, he warned: “A third shot will add to skepticism among people yet to receive one dose that the vaccines help them.”

Together, the new studies indicate overall that vaccines have an effectiveness of roughly 55 percent against all infections, 80 percent against symptomatic infection, and 90 percent or higher against hospitalization, noted Ellie Murray, an epidemiologist at Boston University.

“Those numbers are actually very good,” Dr. Murray said. “The only group that these data would suggest boosters for, to me, is the immunocompromised.”

The apparent reduction in vaccine effectiveness against infection could instead have been caused by increased exposure to the highly contagious Delta variant during a period of unfettered social interactions, she added: “This seems to me like a real possibility, since many early vaccinated were motivated by a desire to see friends and family and get back to normal.”

Dr. Murray said a booster shot would undoubtedly boost immunity in an individual, but the added benefit may be minimal — and obtained just as easily by wearing a mask, or avoiding indoor dining and crowded bars.

The administration’s emphasis on vaccines has undermined the importance of building other precautions into people’s lives in ways that are comfortable and sustainable, and bolstering capacity for testing, Dr. Murray and other experts said.

“This is part of why I think the administration’s focus on vaccines is so damaging to morale,” she added. “We probably won’t be going back to normal anytime soon.”

Before people can begin to receive boosters, the Food and Drug Administration must first authorize a third dose of the vaccines made by Pfizer-BioNTech and Moderna, and an advisory committee of the C.D.C. must review the evidence and make recommendations.

One of the new C.D.C. studies analyzed the effectiveness of vaccines among residents of nearly 4,000 nursing homes from March 1 to May 9, before the Delta variant’s emergence, and nearly 15,000 nursing homes from June 21 to Aug. 1, when the variant dominated new infections in the country.

The vaccines’ effectiveness at preventing infections dropped from about 75 percent to 53 percent between those dates, the study found. It did not evaluate the vaccines’ protection against severe illness.

Nursing homes were required to report the number of immunized residents only after June 6, which “makes comparisons over time very challenging,” Dr. Murray said. “It’s fully possible that the vaccine effectiveness reported here hasn’t actually declined over time.”

The decline in effectiveness also could have resulted from the spread of the Delta variant, Dr. Gounder said.

“It makes sense to give an extra dose of vaccine to vaccinated nursing home residents, but what will have an even bigger impact on protecting those nursing home residents is to vaccinate their caregivers,” she said. Many health aides in long-term care facilities remain unvaccinated.

A second study evaluated data from New York State from May 3 to July 25, when the Delta variant grew to represent more than 80 percent of new cases. The effectiveness of vaccines in preventing cases in adults declined from 91.7 percent to 79.8 percent during that time, the study found. But the vaccines remained just as effective at preventing hospitalizations.

During those weeks, New York recorded 9,675 breakthrough infections — roughly 20 percent of total cases in the state — and 1,271 hospitalizations in vaccinated people, which accounted for 15 percent of all Covid-19 hospitalizations.

Although fully immunized people of all ages got infected with the virus, vaccine effectiveness showed the sharpest drop, from 90.6 percent to 74.6 percent, in people aged 18 through 49 — who are often the least likely to take precautions and the most likely to socialize.

Data from Israel has suggested that immunity against infection has waned in vaccinated adults who are 65 or older. But in the New York data, the effectiveness of the vaccines in that group barely budged.

Adults ages 65 or older were more likely to be hospitalized than other age groups, regardless of vaccination status. But the vaccines did not show a decline in effectiveness against hospitalizations in any of the age groups.

The third study from the C.D.C. found that the vaccines showed 90 percent effectiveness against hospitalizations in the country, “which is excellent,” Dr. Gounder noted.

The vaccines were less protective against hospitalization in immunocompromised people. “But not all immunocompromised persons will respond to an additional dose of vaccine,” Dr. Gounder noted.

To protect these vulnerable individuals, everyone around them should be vaccinated and should continue to wear masks, she added.

The vaccines may appear to be less effective than they did in the trials that led to their authorization because those studies were conducted before the emergence of the Delta variant.

Statistically, the vaccines can appear to lose relative effectiveness as more unvaccinated people become infected, recover and gain natural immunity. And scientists always expected that as more people became vaccinated, the proportions of vaccinated people among the infected would rise.

If preventing infection is the goal, it would be wiser to develop a booster of a nasal spray vaccine, which is better at inducing immunity in the nose and throat, where the virus enters the body, Dr. Gounder said.