As newly published data from Israel suggests that the Pfizer-BioNTech Covid-19 vaccine may be losing effectiveness, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, said officials are reconsidering their current masking guidelines.

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New data from Israel hints at possible decline in vaccine effectiveness

On Thursday, Israel's health ministry released preliminary data suggesting that, while the Pfizer-BioNTech vaccine remains very effective at preventing severe Covid-19, it may be losing effectiveness against milder cases of the disease.

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Specifically, researchers have estimated the vaccine was just 39% effective at preventing infection in Israel in late June and early July, compared to 95% from January to early April. However, the vaccine was more than 90% effective at preventing severe Covid-19 in both time periods.

Experts suggested several possible explanations for the seeming decline, including the possible waning of immune protection in people vaccinated early in the year and the increasing prevalence of coronavirus variants that can partially evade vaccine-induced immunity.

But it's also possible that the apparent decline is a mathematical fluke. Case numbers are much lower in Israel now than they were earlier in the year, so it's increasingly difficult to accurately gauge the degree of protection provided by vaccines.

"I think that data should be taken very cautiously because of small numbers," Eran Segal, a biologist at the Weizmann Institute of Science who is consulting with the Israeli government on vaccines, said.

Ran Balicer, chair of Israel's Covid-19 National Expert Advisory Panel, added that the challenges associated with accurately estimating the vaccine's efficacy are "immense" and that a more detailed analysis of raw data is needed to fully understand what's happening.

Delta variant spreads throughout the US

Even as the early research suggests that vaccine-induced immunity may be faltering, coronavirus case counts are surging in the United States.

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According to the Washington Post, the daily average of confirmed U.S. coronavirus cases has quadrupled in July, rising from around 13,000 per day at the beginning of the month to 43,243 per day now.

According to CDC, the delta variant now accounts for more than 83% of Covid-19 cases in the United States.

"The delta variant is more aggressive and much more transmissible than previously circulating strains. It is one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career," CDC Director Rochelle Walensky said.

According to Fauci, the surge in cases is occurring primarily among those not yet been vaccinated against Covid-19, rather than among the vaccinated. "If you look at the inflection of the curve of new cases … [the growth] is among the unvaccinated. And since we have 50% of the country [who are] not fully vaccinated, that's a problem," he said.

Health officials are reportedly considering changing mask guidelines

In light of these developments, Fauci said updates to current federal masking guidelines are under "active consideration" from CDC.

Sources told the Washington Post last week on condition of anonymity that officials are considering asking Americans to wear masks when both vaccinated and unvaccinated people mix in public places.

"It's fair to say they are reconsidering everything," Marcus Plescia, CMO at the Association of State and Territorial Health Officials, who has spoken with CDC and state officials recently, said. "I think everything's on the table," including changing masking or social distancing guidelines. (Diamond, Washington Post, 7/22; Coleman, The Hill, 7/25; Saric, Axios, 7/25; Duster, CNN, 7/25; Belluck, New York Times, 7/25; Choi, The Hill, 7/25; Zimmer, New York Times, 7/23)