New findings show vaccine effectiveness falls over time - which is why the Biden administration is recommending booster shots

• Vaccinated Americans should get booster shots after eight months, health authorities said Wednesday.
• New data indicates that vaccine protection wanes over time, particularly in the face of Delta.
• The Biden administration had already recommended boosters for some immunocompromised people.

Americans who have been fully vaccinated with Pfizer or Moderna should get a booster shot eight months after their second dose, US health authorities announced Wednesday.

The Biden administration had already recommended boosters for some immunocompromised people, including those who have received an organ transplant or have an advanced HIV infection. The decision to expand that recommendation to all Americans is based on new data indicating that vaccine protection wanes over time - particularly in the face of the Delta variant.

A new CDC study found that the Pfizer and Moderna vaccines lowered the risk of a coronavirus infection by 75% in nursing homes from March to May, before the Delta variant became dominant in the US. But from June to August, after the variant had spread widely, those vaccines only lowered the risk of infection by 53%.

Additionally, a recent study from the Mayo Clinic, which is still awaiting peer review, identified a similar pattern in Minnesota. During the first half of 2021, Pfizer's vaccine lowered the risk of a coronavirus infection by 76%, it found. But in July, when Delta represented the majority of Minnesota's COVID-19 cases, that protection went down to 42%. Moderna's effectiveness also dropped to 76% in July, down from 86% during the first half of the year.

In New York, meanwhile, vaccines were found to be 92% effective at preventing COVID-19 as of May 3 - but by July 25, that effectiveness had gone down to 80%, according to a CDC study released Wednesday.

Other factors are at play in these declines in addition to efficacy drops, though. The researchers suggested that the Delta variant may evade vaccine protection better than its predecessors, and it's also possible that early reports inflated how well the vaccines protected against COVID-19, since Americans were more diligent about masks and social distancing then.

However, vaccines remained just as effective at preventing hospitalizations in New York from May through July: They lowered the risk of hospitalization by 92% to 95% during that time.

"Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout," US health authorities said in a joint statement on Wednesday. "For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability."

Boosters seem to offer more immune protection

Data from Israel, too, suggests that vaccine effectiveness may fall over time. There, Pfizer's vaccine now seems to lower the risk of severe disease and hospitalization by less than 55% among Israelis ages 65 and older who got the shot in January. (Clinical trials previously suggested that Pfizer's shot lowered the risk of severe disease by at least 95% up to six months after the second dose.)

Those findings, however, may be skewed by the fact that older people are more likely to be vaccinated and generally more prone to severe disease. In general, most breakthrough cases in Israel remain mild.

New data from the CDC shows that vaccines are still highly effective against severe disease for up to six months: Pfizer's and Moderna's shots lowered the risk of hospitalization in the US by 84% around three to six months after the second dose, a recent study found.

"While protection against infection may decrease over time, protection against severe disease and hospitalization is currently holding up pretty well," CDC Director Rochelle Walensky said at a White House briefing on Wednesday.

Still, Pfizer's data indicates that a third dose of its vaccine may help maintain a high level of protection against COVID-19 when given 6 to 12 months after the second shot. A study that's still awaiting peer review found that boosters could provide high levels of immune protection against the current variants for at least another six months.

But before booster shots can be rolled out to the public, the FDA still has to authorize them. Assuming that happens, the Biden administration plans to start distributing boosters the week of September 20.

By that point, the first people to get vaccinated in the US - healthcare workers, nursing home residents, and other elderly residents - will have gotten their second dose roughly eight months prior. US health authorities said they would start delivering boosters to residents of long-term care facilities first.

Health officials and other disease experts continue to emphasize, though, that the only way the pandemic will end is for more people to get their first shots.

"The real problem in this country is not that we need to boost the vaccinated - it's that we need to vaccinate the un-vaccinated," Dr. Paul Offit, who sits on the FDA's vaccine advisory committee, told Insider. "That's the problem. Until we do that, we're going to suffer in this country."