Pfizer CEO Albert Bourla on Monday said two doses of the company’s vaccine may not provide strong protection against infection from the omicron Covid variant, and the original shots have also lost some of their efficacy at preventing hospitalization.

Bourla, in an interview at J.P. Morgan’s healthcare conference, emphasized the importance of a third shot to boost people’s protection against omicron.

“The two doses, they’re not enough for omicron,” Bourla said. “The third dose of the current vaccine is providing quite good protection against deaths, and decent protection against hospitalizations.”

Bourla said omicron is a more difficult target than previous variants. Omicron, which has dozens of mutations, can evade some of the protection provided by Pfizer’s original two shots.

“We have seen with a second dose very clearly that the first thing that we lost was the protection against infections,” Bourla said. “But then two months later, what used to be very strong in hospitalization also went down. And I think this is what everybody’s worried about.”

Real-world data from the United Kingdom has found that two vaccine doses are 52% effective at preventing hospitalization 25 weeks after receiving the second shot, according to data from the U.K. Health Security Agency.

Two-doses of Pfizer or Moderna’s vaccines are only about 10% effective at preventing infection from omicron 20 weeks after the second dose, according to the U.K. data.

A booster dose, on the other hand, is up to 75% effective at preventing symptomatic infection and 88% effective at preventing hospitalization, according to the data.

However, Bourla said it’s unclear how long a booster dose will provide protection against Covid. The U.K. Health Security Agency also found that boosters are only 40% to 50% effective against infection 10 weeks after receiving the shot.

“The question mark, it is how long that protection lasts with the third dose,” Bourla said.

The U.S. Centers for Disease Control and Prevention is now recommending that some people with compromised immune systems receive four shots, three primary doses and one booster. Israel has rolled out fourth Pfizer doses for people over the age of 60. Israel found that fourth doses increase protective antibodies fivefold.

Bourla told CNBC earlier Monday that Pfizer will have a vaccine that specifically targets omicron ready in March, though he said it’s not clear whether it will be needed or not.

Bourla also told CNBC more tests need to be done to determine whether or not fourth doses are necessary.