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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.

“There will be more breakthrough” of vaccine-induced immunity, Sigal said. “A good booster probably would decrease your chance of infection, especially severe infection leading to more severe disease. People who haven’t had a booster should get one, and people who have been previously infected should be vaccinated.”

Representatives of Pfizer and BioNTech, makers of the first COVID-19 vaccine cleared in the U.S., didn’t immediately respond to a request for comment.

Omicron’s rapid spread in South Africa has raised concern that the immune protection from vaccination or a previous bout of COVID-19 may be insufficient to stop reinfections or stem a fresh wave of cases and hospitalizations. The World Health Organization has warned omicron could fuel surges with “severe consequences” amid signs that it makes the coronavirus more transmissible.

Still, South Africa’s surge in cases following omicron’s emergence hasn’t overwhelmed hospitals so far, prompting some cautious optimism that the new strain may cause mostly mild illness.

Since South Africa announced the discovery of omicron on Nov. 25, about 450 researchers globally have been working to isolate the variant from patient specimens, grow it in the lab, verify its genomic sequence, and establish methods to test it in blood-plasma samples, according to the WHO.

The work in Sigal’s lab involved testing 14 blood plasma samples collected from a dozen people who had been given a second Pfizer-BioNTech shot about a month earlier to gauge the concentration of antibodies needed to neutralize, or block, the virus. Levels of neutralizing antibodies against omicron were notably higher in a subset of participants who had a bout of COVID-19 about a year earlier, Sigal said.

That’s “promising,” said John Wherry, director of the institute for immunology at the University of Pennsylvania’s Perelman School of Medicine. It likely means an additional dose of the currently available vaccines would boost levels of neutralizing antibodies to omicron, though more data are needed to confirm that, he said.

The results are preliminary and exact levels of immune escape may change, he said. The results, along with those from other labs currently underway, will help determine whether or not existing COVID-19 vaccines need to be altered to protect against omicron. The result of testing at Sweden’s Karolinska Institute is expected imminently, Sigal said.

His laboratory was the first to isolate the beta variant, a strain of the coronavirus that was identified in South Africa in late 2020. Sigal noted that omicron escapes antibody neutralization more readily than beta, which had been considered the most immune evasive of the variants of concern detected previously.

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