Fauci Casts Doubt on Study Suggesting Delta Variant Hit Pfizer COVID Vaccine's Efficacy

Dr. Anthony Fauci has cast doubt on an Israeli study showing the Pfizer vaccine is only 64 percent effective in preventing symptomatic COVID cases amid the spread of the Delta variant.

The Delta variant, also known as B.1.617.2, is what experts consider a variant of concern.

Evidence suggests it could be more transmissible, is harder to neutralize with some antibody treatments, and could affect vaccine efficacy. The U.S. is among the countries where the variant (first detected in India) has spread.

On Monday, Israel's Health Ministry added to fears around the spread of the variant, by saying the effectiveness of the Pfizer vaccine against symptomatic COVID was down from 94.3 percent in May, to 64 percent from June to July.

Pfizer told Newsweek it cannot comment on unpublished data.

The vaccine was, however, found to be 98.2 percent effective in preventing serious illness and hospitalization in May, and down to 93 percent in June.

Israel is among the countries with the most successful COVID vaccine roll-outs, with 59 percent of people fully vaccinated, according to Our World in Data.

On Tuesday, Fauci, President Joe Biden's chief medical adviser, appeared tentative about the findings in an interview with NPR, and said the percentage could be the result of the country testing more people for COVID.

This could make it appear as if the vaccine is less effective.

He said: "We don't have enough data or enough numbers to know what that means.

"It could mean that they just are testing a heck of a lot more people who are asymptomatically infected, and they're seeing a larger percentage of people infected. Hence a diminution in the efficacy of the vaccine."

Fauci told NPR the apparent decline in effectiveness was "something we would certainly want to pay attention to, but I'm not so sure we can make any major conclusions based on the information we have right now."

Dr. Ashish Jha, dean of Brown University's School of Public Health, also addressed the findings.

He wrote in a tweet on Monday: "Speaking to colleagues in Israel, real skepticism about 64 percent number. Best data still suggest mRNA vaccines offer high degree of protection against infection. And superb protection against severe illness. Lets await more data but as of now. If you're vaccinated, I wouldn't worry."

Speaking to colleagues in Israel, real skepticism about 64% number

Best data still suggest mRNA vaccines offer high degree of protection against infection

And superb protection against severe illness

Lets await more data but as of now

If you're vaccinated, I wouldn't worry https://t.co/7g2bPUuclW

— Ashish K. Jha, MD, MPH (@ashishkjha) July 5, 2021

The Israeli study comes after a report released by Public Health England (PHE) in May found the Pfizer vaccine was 88 percent effective against symptomatic COVID, conducted while Delta was spreading in the country.

At the time, Dr. Mary Ramsay, head of immunisation at PHE, said in a statement that the study "provides reassurance" that two doses of the Pfizer vaccine, as well as the AstraZeneca vaccine used in the U.K., "offer high levels of protection against symptomatic disease from the B.1.617.2 [Delta] variant."

She said: "We expect the vaccines to be even more effective at preventing hospitalization and death, so it is vital to get both doses to gain maximum protection against all existing and emerging variants."

Israel's findings come as the U.S. Centers for Disease Control estimates that the Delta variant is now the dominant form in the U.S., making up more than 51 percent of cases.

Update 7/07/21 12:20 p.m. ET: This article has been adapted with comment from Pfizer.

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A stock image shows a health care worker giving a COVID vaccine. There are concerns the Delta variant may make the Pfizer vaccine less effective. Getty Images