Pfizer Inc.’s vaccine provides less immunity to the omicron variant than to other major versions of COVID-19, according to laboratory experiments that indicate a booster may still help stop the highly mutated strain.

Researchers at the Africa Health Research Institute in Durban, South Africa found omicron reduces virus-blocking antibodies by 40-fold in people who have received two doses of the Pfizer-BioNTech SE shot, compared with the strain detected in China almost two years ago.

The loss of immune protection is "robust, but not complete,” Alex Sigal, head of research at the laboratory, said in an online presentation of the first reported experiments gauging the effectiveness of the vaccine against the new variant.