The study found the vaccine was still able to neutralize the virus and there is not yet evidence from trials in people that the variant reduces vaccine protection, the companies said.

Still, they are making investments and talking to regulators about developing an updated version of their mRNA vaccine or a booster shot, if needed.

For the study, scientists from the companies and the University of Texas Medical Branch (UTMB) developed an engineered virus that contained the same mutations carried on the spike portion of the highly contagious coronavirus variant first discovered in South Africa, known as B.1.351. The spike, used by the virus to enter human cells, is the primary target of many COVID-19 vaccines.

Researchers tested the engineered virus against blood taken from people who had been given the vaccine, and found a two- thirds reduction in the level of neutralizing antibodies compared with its effect on the most common version of the virus prevalent in U.S. trials.

Their findings were published in the New England Journal of Medicine (NEJM).

Because there is no established benchmark yet to determine what level of antibodies are needed to protect against the virus, it is unclear whether that two-thirds reduction will render the vaccine ineffective against the variant spreading around the world.

However, UTMB professor and study co-author Pei-Yong Shi said he believes the Pfizer vaccine will likely be protective against the variant.

“We don’t know what the minimum neutralizing number is. We don’t have that cutoff line,” he said, adding that he suspects the immune response observed is likely to be significantly above where it needs to be to provide protection.

That is because in clinical trials, both the Pfizer/BioNTech vaccine and a similar shot from Moderna Inc conferred some protection after a single dose with an antibody response lower than the reduced levels caused by the South African variant in the laboratory study.

Even if the concerning variant significantly reduces effectiveness, the vaccine should still help protect against severe disease and death, he noted. Health experts have said that is the most important factor in keeping stretched healthcare systems from becoming overwhelmed.

More work is needed to understand whether the vaccine works against the South African variant, Shi said, including clinical trials and the development of correlates of protection - the benchmarks to determine what antibody levels are protective.

Pfizer and BioNTech said they were doing similar lab work to understand whether their vaccine is effective against another variant first found in Brazil.

Moderna published a correspondence in NEJM on Wednesday with similar data previously disclosed elsewhere that showed a sixfold drop antibody levels versus the South African variant.

Moderna also said the actual efficacy of its vaccine against the South African variant is yet to be determined. The company has previously said it believes the vaccine will work against the variant.