As the U.S. vaccine rollout stumbles, the C.D.C. tweaked recommendations for second doses, despite a lack of data.
A healthcare worker administers a Covid-19 vaccine in Harlem on Friday. The C.D.C. has updated its guidelines on vaccine doses from multiple manufacturers.
A healthcare worker administers a Covid-19 vaccine in Harlem on Friday. The C.D.C. has updated its guidelines on vaccine doses from multiple manufacturers.Credit…James Estrin/The New York Times

The Centers for Disease Control and Prevention has quietly changed its recommendations for coronavirus immunizations to allow patients to switch the authorized vaccines between the first and second doses in “exceptional situations,” and to extend the interval between doses to six weeks, even though such changes have not been studied in large clinical trials.

The new guidelines were posted on the agency’s website on Thursday with little public notice. With the possibility of vaccine shortages on the horizon and little expectation that supply can be increased before April, the changes may offer a way to vaccinate more people — a high priority for President Biden, who outlined his national Covid-19 strategy on Thursday.

A C.D.C. spokeswoman, Kristen Nordlund, said the agency’s “intention is not to suggest people do anything different, but provide clinicians with flexibility for exceptional circumstances.”

Dr. Anthony S. Fauci, the president’s special adviser for Covid-19, has repeatedly advised against delaying the second dose or making any other changes in vaccination protocol without the data to support them. But on Friday, he seemed open to delaying second doses, at least for short periods.

“What the C.D.C. is saying, sometimes, the situation is stressed where it’s very difficult to be exactly on time,” Dr. Fauci told CNN. “So we’re saying, you can probably do it six weeks later, namely, two additional weeks. Quite frankly, immunologically, I don’t think that’s going to make a big difference.”

Earlier this month, Britain quietly updated its vaccination playbook to allow for a mix-and-match vaccine regimen if the second dose of the vaccine a patient originally received isn’t available, or if the manufacturer of the first shot isn’t known. Some scientists questioned the move at the time, saying Britain was gambling with its new guidance.

In the United States, two vaccines have emergency federal authorization — one by Pfizer and BioNTech, and the other by Moderna — and both rely on the same mRNA technology and call for two doses. Until now, the C.D.C. has strictly adhered to the recommendations from its Advisory Committee on Immunization Practices, which specifically stated that the vaccines were not to be mixed.

The updated C.D.C. guidance still states that the authorized vaccines are “not interchangeable with each other or with other Covid-19 vaccine products.” The agency put the word “not” in bold on its website, and noted that the safety and efficacy of mixing doses has not been studied.

But “in exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available,” the guidelines added, any available mRNA vaccine can be used for the second dose.

With respect to dosing, the guidance says that the second dose should be administered as close as possible to the recommended interval — three weeks for the Pfizer-BioNTech vaccine and four weeks for Moderna. But if that is “not feasible,” the agency wrote, the interval between doses may be extended to six weeks.

The pace of vaccination is critical not just to curbing disease and death, but also to heading off the impact of more infectious forms of the virus. The C.D.C. has warned that one variant, which is thought to be 50 percent more contagious, might become the dominant source of infection in the United States by March.

Although public health experts are optimistic that the existing vaccines will be effective against that variant, known as B.1.1.7, it may drive up the rate of new cases if enough people remain unvaccinated.

At a White House briefing on Thursday — his first since November — Dr. Fauci said that experts are particularly concerned about new variants of the virus in South Africa and Brazil, which have not yet reached the United States. He said vaccines still appear effective against those variants, but the variants may sidestep the immune system to some degree, making it all the more urgent for people to be vaccinated.

“Replicating viruses don’t mutate unless they replicate,” Dr. Fauci said, “and if you can suppress that by a very good vaccine campaign, then you can actually avoid this deleterious effect that you might get from the mutations.”

Federal health officials and corporate executives agree that it will be impossible to increase the immediate supply of vaccines before April because of lack of manufacturing capacity. And the current vaccination effort, which had little central direction under the Trump administration, has so far sown confusion and frustration. Some localities are complaining they are running out of doses, while others have unused vials sitting on shelves.

According to a senior administration official, Pfizer-BioNTech and Moderna are on track to deliver up to 18 million doses a week. Together, they have pledged to deliver 200 million doses by the end of March.

A third vaccine maker, Johnson & Johnson, is due to report the results of its clinical trial shortly. If approved, that vaccine would also help shore up production. If all of that supply were used, the nation could average well over two million shots a day.

In April and afterward, the outlook brightens. Pfizer and Moderna have each committed to supply another 100 million doses by the end of July; the companies may be able to provide even more. A week ago, Pfizer and BioNTech, its German partner, increased their global production target for the year to two billion doses from 1.3 billion doses.

— Sheryl Gay Stolberg The New York Times