Covid Live Updates: Over 200 Million Americans Are Fully Vaccinated

Credit…Alisha Jucevic for The New York Times

Pfizer and BioNTech said Wednesday that laboratory tests suggest that three doses of their coronavirus vaccine offer significant protection against the fast-spreading Omicron variant of the virus.

The companies said that tests of blood from people who received only two doses found much lower antibody levels against Omicron compared with an earlier version of the virus. That finding indicates that two doses alone “may not be sufficient to protect against infection” by the new variant, the companies said.

But the blood samples obtained from people one month after they had received a booster shot showed neutralizing antibodies against Omicron comparable to those against previous variants after two doses, the companies said in a statement.

These experiments, done with blood samples in the lab, cannot say for sure how the vaccines will perform in the real world. Vaccines stimulate a wide-ranging immune response that involves more than just antibodies, but levels of antibodies are the fastest and easiest response to test.

So the experiments offer an incomplete picture of how well the vaccine will protect against severe outcomes from Omicron. Scientists say it could take a month or more to understand the new variant’s threat while Israel, Britain or other countries with sophisticated health surveillance systems gather more data.

Kathrin U. Jansen, a senior vice president and head of vaccine research and development at Pfizer, described the study as small and preliminary. Researchers tested a total of 39 blood samples from individuals who had received either their second or third shot within the previous month.

The results seemed to underscore the importance of boosters in combating infection. President Biden went out of his way to draw attention to the findings on Wednesday, telling reporters while they were only based on a lab report, they were “very, very encouraging.”

“If you get the booster, you’re really in good shape,” he said. According to federal data, the United States has surpassed more than 200 million fully vaccinated people.

Dr. Albert Bourla, the chairman and chief executive officer of Pfizer, said while two doses may still prevent severe disease from Omicron, “it’s clear from these preliminary data that protection is improved with a third dose of our vaccine.”

Dr. Ugur Sahin, the chief executive officer of BioNTech, Pfizer’s German partner, said that “a third dose could still offer a sufficient level of protection from disease of any severity” from the variant.

The companies suggested that Omicron would not significantly diminish the power of T cells — another, more lasting part of the immune system’s response to the coronavirus. Researchers identified parts of Omicron that can be recognized by the T cells produced after vaccination. Most did not contain any mutations.

Company officials did not data showing how T cells actually perform against the variant; nor did they release the other data, instead summarizing the findings in a news release.

In the interview, Dr. Jansen said it is “very important” that the parts of Omicron 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 also 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 he said there is no evidence that the vaccines have become less effective at preventing severe disease.

“In all likelihood, two doses of an mRNA vaccine will protect you against serious illness,” he said.

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 most people getting booster shots.

Dr. Peter Hotez, a vaccine expert with the Baylor College of Medicine, called the results “really good news.” But he noted that Pfizer’s experiments only measured the levels of neutralizing antibodies one month after the third shot, saying he was concerned by other research suggesting that those levels might drop off later, more sharply than expected.. Dr. Jansen said researchers used those blood samples because they had a very extensive database on them.

The results come one day after a preliminary report on laboratory experiments in South Africa also found Omicron seemed to dull the power of two doses of Pfizer’s vaccine. The South African experiments did not try to evaluate how well three doses worked. The study found antibodies produced by people with two doses of vaccine were much less successful at fending off infection from Omicron than from previous variants.

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 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 young people infected, a low percentage of people vaccinated, and a high rate of H.I.V., which can damage the immune system. And others cautioned against drawing conclusions from scattered early reports.

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

“We will be able to switch overnight production,” Dr. Bourla said. “There’s not going to be a need to start producing new machinery, new equipment, new formulations.”

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

Noah Weiland, Carl Zimmer and Benjamin Mueller contributed reporting.

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