A two-dose course of the Pfizer-BioNTech COVID-19 vaccine may not be enough to effectively combat the new omicron coronavirus variant, but a booster shot may do the trick, two early lab studies suggest. 

The omicron variant is able to evade much of the protective response produced by the vaccine but can’t escape completely, a study from South Africa finds. Pfizer, which has conducted its own study, says its evidence shows that a third shot will boost vaccine efficacy significantly.

Pfizer tested blood samples from vaccine recipients three weeks after they received a second dose or one month after receiving a third dose. The third dose significantly increased the neutralizing antibody levels against the omicron strain, the company said in a statement released Wednesday.

“Although two doses of the vaccine may still offer protection against severe disease caused by the omicron strain, it’s clear from these preliminary data that protection is improved with a third dose of our vaccine,” said Albert Bourla, CEO.

Three doses against omicron are “almost equivalent” to the effectiveness of two doses against the original variant, Bourla told NBC Wednesday on its “Today” show.

The other study from South Africa, which has yet to be peer-reviewed, involved blood samples from 12 volunteers, six of whom had been vaccinated with an initial series of the Pfizer vaccine, although not boosted, and six who had previously weathered a bout with COVID-19.

Investigators found a significant drop in the response of vaccine-produced antibodies to omicron when compared with the response produced to delta and the original coronavirus variant. But samples from five participants who had prior COVID-19 infections showed evidence of “very strong neutralization” of the omicron variant, similar to what would be found soon after vaccination. 

There also was evidence that a third shot of the Pfizer vaccine would help add needed protection, the investigators reported.

“Previous infection, followed by vaccination or booster is likely to increase the neutralization level and likely confer protection from severe disease in omicron infection,” they concluded.

The findings support other early evidence that the currently available vaccines may have only modest efficacy in the face of the new variant. Protection from two initial doses of the Pfizer and Moderna vaccines is likely to be “marginal,” one expert told Stat. 

“For people infected and then vaccinated and for people who got booster shots the situation likely looks much better,” Florian Krammer, Ph.D., a vaccinologist at Mount Sinai’s Icahn School of Medicine, told the medical news outlet. His bigger concern is the fate of unvaccinated people, he said.