Laboratory experiments have found that Omicron variant of Covid-19 virus 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, said a study.

Published online on Tuesday, the study 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 hospitalisation or death from Omicron.

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 1/40th 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.

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.

Pfizer and Moderna have said 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.