A report out of South Africa offered a first glimpse at how vaccinated people might fare against the fast-spreading Omicron variant of the coronavirus.

Laboratory experiments found that Omicron seems to dull the power of the Pfizer-BioNTech vaccine, but also hinted that people who have received a booster shot might be better protected.

The study, published online on Tuesday, found that antibodies produced by vaccinated people were much less successful at keeping the Omicron variant from infecting cells than other forms of the coronavirus.

Scientists said the results were somewhat worrisome, but no cause for panic. The data suggests that vaccinated people might be vulnerable to breakthrough infections with Omicron, which is spreading rapidly in South Africa and has appeared in dozens of countries around the world.

But vaccines stimulate a wide-ranging immune response that involves more than just antibodies. So these experiments offer an incomplete picture of how well the vaccine protects against hospitalization or death from Omicron.

“While I think there’s going to be a lot of infection, I’m not sure this is going to translate into systems collapsing,” Alex Sigal, a virologist at the Africa Health Research Institute in Durban, South Africa, who led the research, said in an interview. “My guess is that it’ll be under control.”

Dr. Sigal and his colleagues worked at breakneck speed over the past two weeks to grow the virus and then test antibodies against it. “If I don’t die from the virus, I’ll die of exhaustion,” he said.

Originally, Dr. Sigal feared that vaccines might not provide any protection at all. It was possible that the Omicron variant had evolved a new way of entering cells, which would have rendered antibodies from vaccines useless. “Then all our efforts would be trash,” he said.

Fortunately, that proved not to be the case.

Dr. Sigal and his colleagues used antibodies from six people who received the Pfizer vaccine without ever having had Covid-19. They also analyzed antibodies from six other people who had been infected before getting the Pfizer vaccine.

The researchers found that the antibodies from all of the volunteers performed worse against Omicron than they did against an earlier version of the coronavirus. Overall, their antibodies’ potency against Omicron dropped dramatically, to about one-fortieth of the level seen when tested with an earlier version of the virus. That low level of antibodies may not protect against breakthrough Omicron infections.

Theodora Hatziioannou, a virologist at Rockefeller University who was not involved in the research, said that number was not surprising. “It’s more or less what we expected,” she said.

The results could help explain some high-profile superspreading events caused by Omicron. At an office Christmas party in Norway, the virus seems to have infected at least half of 120 vaccinated attendees.

Dr. Sigal announced the results on Twitter Tuesday afternoon.

His team found a distinct difference between the two sets of volunteers. The antibodies from the six uninfected vaccinated people were very weak against Omicron. But among the volunteers who had Covid-19 before vaccination, five out of six still produced fairly potent responses.

One reason for the difference is that people who are vaccinated after an infection produce higher levels of antibodies than do people who were not infected.

Dr. Sigal said the experiments will not be able to say much about how well boosters protect against Omicron until researchers directly test antibodies from people who have received them. But he suspected that the increased level of antibodies would provide good protection. “The more you’ve got, the better you’ll be,” he said.

Kristian Andersen, an infectious disease researcher at the Scripps Research Institute in La Jolla, Calif., who was not involved in the new study, agreed that booster shots were likely to help fend off the new variant.

“I expect boosters to restore better levels of protection,” he said. “And, importantly, early clinical data from South Africa suggest that immunity — whether from vaccines or prior infections — is still effective in preventing the more severe forms of Covid-19.”

Dr. Hatziioannou was less certain about boosters. She and her colleagues are running experiments on antibodies from boosters to test whether they will produce the same robust protection seen in people who got vaccines after infection. “I want to say yes, but we have to wait,” she said.

Pfizer and Moderna have said that they were testing their vaccines against Omicron, and that they would be able to produce vaccines specifically tailored to the variant in roughly three months.

Jesse Bloom, a virologist at the Fred Hutchinson Cancer Research Center, said that the study reinforced the need to accelerate the development of Omicron-specific shots. Even though there remains some uncertainty about how widely the variant will spread, he said, the best way of restoring protection against Omicron will be to give people a vaccine containing Omicron’s genetic information.

“Given the very large drop in neutralizing antibody titers that are seen here with Omicron,” he said, “certainly in my view it would merit pushing forward as fast as possible with making Omicron-specific vaccines, as long as it seems like there’s a possibility it could spread widely.”