CORRECTED-Pfizer says South African variant could significantly reduce protective antibodies

(Corrects headline and first paragraph in Feb 17 story to showthe reduction was in the protective antibodies elicited by thevaccine, not the protection of the vaccine overall)

By Michael Erman

Feb 17 (Reuters) - A laboratory study suggests that theSouth African variant of the coronavirus may reduce protectiveantibodies elicited by the Pfizer Inc/BioNTech SEvaccine by two-thirds, and it is not clear if the shotwill be effective against the mutation, the companies said onWednesday.

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

Still, they are making investments and talking to regulatorsabout developing an updated version of their mRNA vaccine or abooster shot, if needed.

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

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

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

Because there is no established benchmark yet to determinewhat level of antibodies are needed to protect against thevirus, it is unclear whether that two-thirds reduction willrender the vaccine ineffective against the variant spreadingaround the world.

However, UTMB professor and study co-author Pei-Yong Shisaid he believes the Pfizer vaccine will likely be protectiveagainst the variant.

"We don't know what the minimum neutralizing number is. Wedon't have that cutoff line," he said, adding that he suspectsthe immune response observed is likely to be significantly abovewhere it needs to be to provide protection.

That is because in clinical trials, both the Pfizer/BioNTechvaccine and a similar shot from Moderna Inc conferredsome protection after a single dose with an antibody responselower than the reduced levels caused by the South Africanvariant in the laboratory study.

Even if the concerning variant significantly reduceseffectiveness, the vaccine should still help protect againstsevere disease and death, he noted. Health experts have saidthat is the most important factor in keeping stretchedhealthcare systems from becoming overwhelmed.

More work is needed to understand whether the vaccine worksagainst the South African variant, Shi said, including clinicaltrials and the development of correlates of protection - thebenchmarks to determine what antibody levels are protective.

Pfizer and BioNTech said they were doing similar lab work tounderstand whether their vaccine is effective against anothervariant first found in Brazil.

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

Moderna also said the actual efficacy of its vaccine againstthe South African variant is yet to be determined. The companyhas previously said it believes the vaccine will work againstthe variant.(Reporting by Michael Erman; Additional reporting by JulieSteenhuysen in Chicago; Editing by Bill Berkrot)