CDC on Wednesday released three new studies in its Morbidity and Mortality Weekly Report (MMWR) that show the effectiveness of Covid-19 vaccines against infection has waned, but that the vaccines remain effective at preventing hospitalizations and deaths.

Your top resources on the Covid-19 vaccines

Details on the studies

One study looked at how the vaccines developed by Pfizer-BioNTech and Moderna performed among the residents of nearly 4,000 nursing homes between March 1 and May 9, before the delta variant emerged, as well as in almost 15,000 nursing homes from June 21 to Aug. 1, after the delta variant arrived.

'The Never Again Plan': Moderna CEO Stéphane Bancel wants to stop the next Covid-19—before it happens

It found that the vaccines' protection against infection dropped from 75% before the delta variant arrived to 53% after. The study did not specify whether a Covid-19 case was asymptomatic, symptomatic, or severe.

A second study looked more specifically at the vaccines' effectiveness against severe Covid-19. In reviewing data from 21 hospitals from March 11 to July 14, the study found no significant waning of effectiveness over time, with the vaccines consistently providing about 85% protection against hospitalization. However, CDC Director Rochelle Walensky noted the study didn't include many patients infected with the delta variant. Of the 454 coronavirus samples that underwent whole genome sequencing, only 16.3% were the delta variant, while 53.3% were the alpha variant.

In the third study, researchers looked at data collected in New York between May 3 and July 25. It found the vaccines' effectiveness against infection dropped from 91.7% to 79.8%. However, effectiveness against hospitalization remained relatively stable.

In a press briefing, Surgeon General Vivek Murthy said the data on waning vaccine effectiveness was concerning.

"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," he said.

Why some experts question the need for boosters

Motivated in part by the MMWR study data, Biden administration announced on Wednesday that booster shots will be available to those fully vaccinated with an mRNA vaccine eight months after their second shot beginning the week of Sept. 20, pending approval by FDA and a recommendation from CDC.

Should you mandate a Covid-19 vaccine for your staff? Ask these 5 questions first.

In a joint statement, senior health officials at HHS said data "make[s] it very clear that protection against SARS-Cov-2 infection begins to decrease over time following the initial doses of vaccination," adding that the spread of the delta variant is leading to "reduced protection against mild and moderate disease."

However, some experts said the MMWR data suggests booster shots aren't necessary for everyone.

Anna Durbin, a vaccines researcher at Johns Hopkins University, said the vaccines are still effective at preventing hospitalization, severe Covid-19, and death. "We cannot keep [boosting] and say: 'We're going to prevent colds in everybody,'" she said. The decision to offer booster shots to the general public is a result of a "tidal wave building that's based on anxiety," she added. "And I don't think it's based on scientific evidence that a booster is needed."

"I mostly care about hospitalizations, I don't care about infections because this is not what we're using vaccines for," Jennifer Nuzzo, an epidemiologist and senior scholar at the Johns Hopkins Center for Health Security, said. "We're not trying to stop infections, and there's no evidence that a third booster will stop infections."

Still, Anne Rimoin, an epidemiologist at the University of California, Los Angeles, suggested that more research is needed into how often vaccinated individuals with breakthrough infections spread the virus to others—some of whom might themselves be unvaccinated.

Some experts drew a distinction between booster shots for the general public and those for immunocompromised individuals. So far, CDC data shows that fully vaccinated immunocompromised people account for 40% to 44% of breakthrough infections that require hospitalization.

CDC's studies "support giving additional doses of vaccine to highly immunocompromised persons and nursing home residents, not to the general public," Céline Gounder, an infectious disease specialist at Bellevue Hospital Center, 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" to start giving the public booster shots, she argued. "We'll be better protected by vaccinating the unvaccinated here and around the world." (Owens, Axios, 8/19; Mandavilli, New York Times, 8/198; Mandavilli, New York Times, 8/17; Weixel, The Hill, 8/18; Fiore, MedPage Today, 8/18; Branswell, STAT News, 8/18; Rothschild, Axios, 8/18; Harris, STAT News, 8/18)