Pfizer and BioNTech said Wednesday preliminary lab results show three doses of their COVID-19 vaccine are highly effective against the omicron variant.

The vaccine makers said a booster shot appears to increase antibody protection 25-fold compared to two shots. The antibody response was comparable to what was seen after two doses against the original strain of the virus that was first detected in Wuhan, China, they said.

Pfizer and BioNTech, however, did say they believe the initial two doses still protect people from severe illness. 

In the United States, individuals are only eligible to receive a booster shot six months after their second Pfizer or Moderna shots and two months after their first Johnson & Johnson shot. The FDA now allows for mixing-and-matching of booster shots, meaning people do not need to receive the same brand they initially did.

“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 Chairman and CEO Albert Bourla said in a news release. “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.”

The companies said they will continue collecting more lab and real-world data on how omicron might impact its vaccine’s effectiveness. They have begun developing an updated vaccine that targets the new variant in the event that is needed. They say the first batches can be ready for deliveries within 100 days, pending regulatory approval. 

Much remains a mystery about omicron, which was first identified in South Africa, including whether it can evade existing vaccines. Health officials are also still waiting for enough data to determine if it is more contagious and results in more severe illness than earlier versions of COVID-19.

On Tuesday, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and President Joe Biden’s chief medical adviser, said early indications so far point to omicron being more transmissible not as severe.