Laboratory tests suggest that a booster shot of the vaccine may offer protection against the Omicron variant of the coronaviruses.
The companies said that tests of blood from people who had only two doses of the vaccine found lower levels of the antibodies that protect against Omicron than against an earlier version of the virus. Two doses may not be enough to protect against infections by the new variant, the companies said.
The companies only examined about 39 samples and the findings were a bit hopeful at a time of renewed uncertainty. Almost all of the deaths in the United States are due to the Delta variant and health departments are identifying close to 100,000 cases a day.
The companies summarized their findings in a news release. A preliminary report on laboratory experiments in South Africa found that Omicron seemed to dull the power of two doses of the Pfizer vaccine.
The Omicron variant has been found in about 20 American states so far, with cases rising much faster in parts of South Africa and Europe. It may move twice as fast as Delta.
There are more children testing positive for the coronaviruses in South Africa after being admitted for other reasons, suggesting increased community transmission there. Christmas and New Year's Eve events are being canceled around the world due to 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.
Mr. Biden said that if you get the booster, you are in good shape. The United States has more than 200 million people who have been fully vaccined, but only 50 million have gotten a booster dose.
The study of blood samples in a laboratory is not proof of how the vaccines will perform in the real world. The first line of defense against infections is antibodies, but they are only part of the immune system's response. The results come first because they are the easiest to measure.
Kathrin U. Jansen, a senior vice president and the head of vaccine research and development at Pfizer, said that you have to start somewhere.
It could take a month or more for scientists to understand the new variant. Israel, Britain and other countries with sophisticated health systems will have more data on whether Omicron will overtake Delta and how the vaccines will hold up against it.
The Pfizer-BioNTech results showed the importance of boosters in fighting infections. The blood samples obtained from people who had received a booster shot contained neutralizing Omicron at levels comparable to the original variant after two doses, according to a statement from Pfizer.
The results were called "really good news" by Dr. Peter Hotez, a vaccine expert with the Baylor College of Medicine. He is concerned that the surge of virus-blocking antibodies provided by a booster may be short-lived.
The World Health Organization said on Wednesday that it was too early to conclude whether the vaccines were less effective against Omicron or whether the emergence of the variant necessitated a booster shot for most people.
The chief executive of Pfizer and the chief executive of BioNTech both said that the study shows that a third dose strengthens protection. Dr. Sahin said three doses could still offer a sufficient level of protection from disease of any severity. Both companies have made a lot of money from the global demand for their shots.
The companies said that Omicron wouldn't affect the power of T-cells. The parts of Omicron that could be seen by T-cells were identified by researchers. Most did not have any changes.
The parts of the Omicron variant targeted by virus-killing cells were mostly unchanged from previous versions. She said that it gave her comfort that there would be enough T-cell responses to prevent the worst outcomes.
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 wanted to sound a note of reassurance.
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There are new Covid treatments. The pills for treating Covid-19 should be available soon. Scientists say we will need an arsenal of drugs to deploy against new variant if they erode the protection of existing vaccines.
He said that the virus has become less neutralizable due to its evolution. Two doses of the vaccine will protect you against serious illness.
The laboratory experiments in South Africa didn't try to evaluate how well three doses worked. They found that people with two doses of vaccine were less successful at protecting themselves from Omicron than they were from previous versions. The study looked at six people who had received the Pfizer vaccine and six people who had been exposed to Covid-19.
The Biden administration is bracing for a new flood of winter cases from Omicron, as the Delta variant is still overwhelmingly dominant in the United States.
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.
Anthony S. Fauci, the U.S. government's top infectious disease expert, said at a White House briefing Tuesday that they shouldn't make a definitive conclusion before the next couple of weeks.
He said that early reports from South African medical officials presented a somewhat hopeful picture of Omicron. Patients with the coronaviruses are less ill than those who have been treated before, and other hospitals are seeing the same trends, as reported by researchers at a major hospital complex.
Dr. Fauci said that hospital stays were shorter and patients needed less oxygen. The ratio of hospitalizations per number of new cases may be less severe.
He noted that South Africa has a high proportion of young people who are infectious, a low percentage of people who are vaccined, and a high rate of H.I.V., which can damage the immune system. Some cautioned against drawing conclusions from scattered reports.
Dr. Bourla said in an interview last week that the company had begun developing a vaccine that could be produced in mass quantities within 95 days. Moderna is on the same path.
Dr. Bourla said that Pfizer would be able to switch production over night if needed.
He noted that Pfizer had developed two other prototypes in response to new variants, but neither proved necessary because the original vaccine worked against the virus.
Benjamin, Noah, and Carl were involved in the reporting.
