U.S. Government Scientists Skeptical of One-Shot Regimen for Pfizer, Moderna Covid Vaccines

The FDA late last year approved a two-dose regimen for vaccines from

Moderna Inc.

and from a partnership of

Pfizer Inc.

and

BioNTech

SE. More recently it approved use of a one-dose regimen for a vaccine from

Johnson & Johnson.

Some scientists and lawmakers have called for shifting to a one-dose regimen for all the vaccines, citing preliminary studies showing one shot can be effective. They contend shifting to one shot will allow the U.S. to accelerate the pace of vaccinations.

In a March 2 letter to acting Health and Human Services Secretary

Norris Cochran,

seven physician members of Congress urged the department “to consider issuing a revised emergency use authorization as soon as possible” that might lead to single-dose use of the

Pfizer

and Moderna vaccines.

“Last week, the U.S. passed a sobering milestone of over 500,000 deaths related to COVID-19,” said the letter, signed by lawmakers including

Rep. Andy Harris

(R, Md.) and

Rep. Gregory F. Murphy,

(R., N.C.). “These are staggering statistics, and anything we can do to help prevent further tragedy—to further protect the public health and safety of the American people—should be fully employed.”

In interviews, senior government scientists at the FDA and the National Institutes of Health said such a shift isn’t warranted, saying the evidence used to approve the Pfizer and Moderna vaccines was based on two doses.

These scientists said one dose may offer short-term protection, but the longer-term protection is a question mark.

“You would be flying blind to just use one dose,” said one senior scientist and adviser to President

Biden.

“If you’re going to do something else other than follow the studies shown to the FDA, show me that this one-shot effect is durable.”

Another senior U.S. government doctor said the durability of the vaccination is especially important when more-resistant strains of Covid-19, including those from the U.K. and South Africa, are appearing in the U.S.

“We think it’s best to get people to as high a level of immunity as possible,” the doctor said.

The doctor added that the pace of vaccinations is accelerating with the recent decision by

Merck

& Co. to help produce the J&J vaccine.

“We’re going to have a good supply of vaccines very soon,” the doctor said.

Representatives of Pfizer and Moderna didn’t immediately respond to requests for comment Saturday. Pfizer has previously said it doesn’t have data regarding the single-dose approach, and Moderna has previously said it isn’t studying the issue.

Paul A. Offit

of the Children’s Hospital of Philadelphia, who served on the FDA advisory panel that recommended the use of the Pfizer and Moderna vaccines, said those clinical trials “found a level of neutralizing antibodies [with one dose] that was significantly less than what they got with two doses.”

The FDA advisory panel’s chairman,

Arnold Monto,

also said the two-shot regimen is best for the Pfizer and Moderna vaccines. Dr. Monto, a public-health doctor at the University of Michigan, stressed the need for two doses to counter the Covid-19 variants.

“We’ve got information on a two-dose strategy,” said Dr. Monto. “We need high antibody levels from those doses to deal with the variants.”

Others holding fast with similar views in the U.S. government are prominent infectious-disease doctor

Anthony Fauci

and

Andy Slavitt,

a senior White House adviser for Covid-19 response. Mr. Slavitt said it would be a mistake for the U.S. government to be persuaded by just one study.

University of Minnesota epidemiologist

Michael Osterholm

said in testimony Thursday before the Minnesota legislature that the U.S. should consider delaying second doses so more people can get first shots.

“We could get more of our over-65 group vaccinated,” he said. “I think the data will support that actually is a very effective way to go.”

Two weeks ago, researchers in Israel reported that one dose of the Pfizer vaccine was 85% effective in preventing symptomatic disease 15 to 28 days after inoculation.

In the U.K, the government has opted to stretch vaccine supplies by delaying a second dose by up to 12 weeks in a bid to reach more people.

British researchers released preliminary data in recent days saying that either of two vaccines—from Pfizer and from

AstraZeneca

PLC—reduced the risk of hospitalization among people older than 70 years old by 80%, compared with people of similar ages without vaccination.

AstraZeneca is still conducting a U.S. study of its vaccine, which hasn’t yet gained authorization from the FDA.

Write to Thomas M. Burton at tom.burton@wsj.com