Publisher Health

WASHINGTON — Pfizer and BioNTech said Wednesday that laboratory tests suggest a booster shot of their coronavirus vaccine offers significant protection against the fast-spreading Omicron variant of the virus.

The companies said that tests of blood from people who had received only two doses found much lower levels of antibodies protecting against Omicron than against an earlier version of the virus. That suggests that two doses “may not be sufficient to protect against infection” by the new variant, the companies said.

While limited in scope — to get fast results, the companies examined only about 39 samples — the findings provided a bit of hopeful news at a time of renewed uncertainty. Health departments are identifying close to 100,000 cases a day, hospitalizations are ticking up and deaths are again on the rise in the United States, almost all due to the Delta variant.

The companies summarized their findings in a news release and did not release any data. Their study came on the heels of a preliminary report on laboratory experiments in South Africa that also found Omicron seemed to dull the power of two doses of the Pfizer vaccine.

The CDC had recorded 43 cases of Omicron infection, most of them mild, in about 20 American states as of Wednesday, the Associated Press reported. Cases are rising much faster in parts of South Africa and Europe. Early modeling and analysis suggest that it may move twice as fast as Delta.

In South Africa, where Omicron already appears to be dominant, two large hospitals are reporting more children testing positive for the coronavirus after being admitted for other reasons, suggesting increased community transmission there. Around the world, cities are canceling Christmas and New Year’s Eve events amid unresolved questions about the transmissibility and virulence of the new variant.

President Biden went out of his way to draw attention to Pfizer-BioNTech’s findings on Wednesday, calling them “very, very encouraging” and saying they showed that the vaccines remain a bulwark against the virus.

“If you get the booster, you’re really in good shape,” Mr. Biden said. According to federal data, the United States has more than 200 million fully vaccinated people, but only about 50 million have gotten a booster dose.

But Pfizer-BioNTech’s study of blood samples in a laboratory is not proof of how the vaccines will perform in the real world. While antibodies are the first line of defense against infection, they are only part of a wider-ranging and powerful response by the immune system. Because antibodies are the fastest and easiest part to measure, those results typically come first.

“You have to start somewhere,” said Kathrin U. Jansen, a senior vice president and the head of vaccine research and development at Pfizer, while awaiting the results of more complex studies and “real-world evidence that tells you what we need to know.”

Scientists say it could take a month or more to really understand the new variant’s threat. By then, they say, Israel, Britain or other countries with sophisticated health surveillance systems will have gathered more data on whether Omicron will overtake Delta and how the vaccines will hold up against it.

The Pfizer-BioNTech results seemed to underscore the importance of boosters in combating infection. The blood samples obtained from people who had received a booster shot contained antibodies neutralizing Omicron at levels comparable to those combating the original variant after two doses, Pfizer’s statement said.

While calling the results “really good news,” Dr. Peter Hotez, a vaccine expert with the Baylor College of Medicine, noted that researchers only measured the levels of neutralizing antibodies one month after a booster injection. He is concerned, he said, that the surge of virus-blocking antibodies provided by a booster may be short-lived.

The World Health Organization, which has long resisted broad rollouts of booster shots amid severe vaccine shortages in poorer nations, said on Wednesday that it was too early to conclude whether the vaccines were significantly less effective against Omicron or whether the emergence of the variant necessitated a booster shot for most people.

Both Dr. Albert Bourla, the chief executive officer of Pfizer, and Dr. Ugur Sahin, the chief executive of BioNTech, said that while two doses may still prevent severe disease from Omicron, the study demonstrates that a third strengthens protection. Dr. Sahin said three doses “could still offer a sufficient level of protection from disease of any severity” caused by the variant. Like other vaccine manufacturers, both companies have profited hugely from the global demand for their shots.

The companies suggested that Omicron would not significantly diminish the power of T-cells, which kill off infected cells. Researchers identified parts of Omicron that could be recognized by the T-cells produced after vaccination. Most did not contain any mutations.

Dr. Jansen said it was “very important” that the parts of the Omicron variant targeted by virus-killing cells were mostly unchanged from previous variants. “It gives comfort that you will have sufficient T-cell responses to prevent the worst outcomes,” she said.

Dr. Paul Offit, a vaccine expert with Children’s Hospital of Philadelphia and a member of the Food and Drug Administration’s vaccine advisory committee, said he, too, wanted “to sound a note of reassurance.”

“The virus has mutated to the point that it has become less neutralizable by antibodies,” he said. “But in all likelihood, two doses of an mRNA vaccine will protect you against serious illness.”

The separate laboratory experiments in South Africa did not try to evaluate how well three doses worked. But they found that antibodies produced by people with two doses of vaccine were much less successful at fending off infection from Omicron than infection from previous variants. That study looked at six people who had received the Pfizer vaccine without ever having had Covid-19, and six people who had been infected before getting the vaccine.

Omicron has now spread to dozens of countries, and while the Delta variant is still overwhelmingly dominant in the United States, the Biden administration is bracing for a new flood of winter cases from Omicron.

While lab results are one indicator of what comes next, administration officials say it will take a month or two to get more definitive real-world data from countries like Israel that carefully track every patient.

“We shouldn’t be making any definitive conclusions, certainly not before the next couple of weeks,” Dr. Anthony S. Fauci, the U.S. government’s top infectious disease expert, said at a White House briefing Tuesday.

He said early reports from South African medical officials presented a somewhat hopeful picture of Omicron’s impact. Researchers at a major hospital complex in Pretoria reported this week that patients with the coronavirus are significantly less ill than those they have treated before, and that other hospitals are seeing the same trends.

“We are not seeing a very severe profile of disease,” Dr. Fauci said, adding that hospital stays were shorter and patients required less oxygen. “It might be — and I underscore might — be less severe, as shown by the ratio of hospitalizations per number of new cases.”

But he noted that South Africa’s population differed from that of the United States, with a high proportion of infected young people, a low percentage of vaccinated people, and a high rate of H.I.V., which can damage the immune system. Others cautioned against drawing conclusions from scattered early reports.

In an interview last week, Dr. Bourla said the company had begun developing a version of its vaccine targeting Omicron right after Thanksgiving and could produce it in mass quantities within 95 days. Moderna is on much the same path.

Dr. Bourla said that if necessary, Pfizer would be able to switch production “overnight,” adding: “There’s not going to be a need to start producing new machinery, new equipment, new formulations.”

He noted that Pfizer had developed two other prototypes in response to new variants, but neither had proved necessary because the original vaccine worked against the virus’s mutations.

Benjamin Mueller, Noah Weiland and Carl Zimmer contributed reporting.