Pfizer Inc. and BioNTech SE’s COVID-19 vaccine stimulated roughly two-thirds lower levels of neutralizing antibodies against the South African variant of the coronavirus in a lab study.

The Pfizer results are part of tests of its vaccine against a lab-created virus that had all the mutations found in the South African variant, which is thought to spread faster than earlier versions. The study released Wednesday showed reduced neutralization of the South Africa-like virus by blood from people who had been immunized with the Pfizer-BioNTech vaccine. The companies believe their vaccine will still work against the variant.

All the blood samples from vaccinated patients still showed significant levels of neutralizing antibody against the South Africa variant, the researchers said. The study was conducted by Pfizer, BioNTech and researchers at the University of Texas Medical Branch at Galveston and published in the New England Journal of Medicine.

A separate study by researchers from the National Institute of Allergy and Infectious Diseases and Moderna Inc. also looked at how the company’s vaccine performed against the South Africa variant in the lab, and found a significant reduction in coronavirus-fighting antibodies. These neutralizing antibodies were reduced by a factor of 6.4, compared with levels produced against earlier versions of the virus, when the full group of South Africa mutations were examined.

What the lab data means in practice remains to be seen. “It is unclear what effect a reduction in neutralization by approximately two-thirds would have” on protection from COVID-19 caused by the South Africa variant, the Pfizer study concluded.

The mutant virus has become a stumbling block for vaccine campaigns, with South Africa halting the rollout of AstraZeneca Plc’s shot after a study showed it didn’t work as well in preventing mild illness involving the variant first identified there. AstraZeneca is working to revise its vaccine by fall, while Moderna Inc. is working on booster shots.

Meanwhile, a shot from Johnson & Johnson did prevent hospitalizations and deaths from COVID in a large study -- but was less effective at preventing infections in South Africa than elsewhere. A vaccine developed by Novavax Inc. was also less effective in South Africa in a study.