Pfizer and BioNTech have started a study comparing their Omicron-based Covid-19 vaccine to the original vaccine, the companies said in a statement on Tuesday.

If regulators were to decide to wait for data from such a study, it could mean it will be many months before an Omicron-specific vaccine would be available.

Pfizer said it expects initial findings from the study, which will enroll 1,420 volunteers, to be available during the first half of 2022.

However, it is not exactly clear what data regulators such as those at the Food and Drug Administration would require before authorizing an Omicron-specific vaccine, or what requirements the Centers for Disease Control and Prevention will have for recommending one.