Three doses of Pfizer and BioNTech’s vaccine provide a high level of protection against the omicron variant of the virus that causes Covid-19, the companies announced Wednesday.

A booster shot of the vaccine increases antibody protection 25-fold compared with the initial two-dose series, according to a preliminary lab study. A third shot shows virus-fighting abilities comparable with the 95% protection provided by two doses against the original strain of the virus.

The companies said two doses of the vaccine showed a significant reduction in the ability of antibodies to target and neutralize omicron, though they may still protect against severe disease.

“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,” Pfizer CEO Albert Bourla said in a joint statement with BioNTech.

“Ensuring as many people as possible are fully vaccinated with the first two dose series and a booster remains the best course of action to prevent the spread of COVID-19,” Bourla said.

Bourla, in an interview Wednesday on CNBC’s “Squawk Box,” said the lab test was against a synthetic, lab-produced copy of the virus. He said Pfizer and BioNTech are working to reproduce the same results with the real virus in a week or two. Those results would be more accurate, he said.

“The ultimate proof is coming from real-world data,” Bourla told CNBC. He said the companies need to know how many breakthrough infections occur and the severity of the cases in people who have received boosters compared with those who have had the two-dose initial series.

Bourla said real world data will show how long the booster dose provides protection against omicron. “At a certain point I think we will need a fourth dose,” he told CNBC.