The FDA said on Wednesday that some vials of the Pfizer coronavirus vaccine, which began being administered to health care workers earlier this week, are overfilled with doses. These doses, the FDA says, can be used for additional vaccinations, which will help expand the supply.

Each vial of the Pfizer vaccine contains five doses. But after the vaccine is thawed and administered, those administering the vaccine discovered it is possible to get a sixth, or even seventh dose from the vaccine.

This discovery could potentially allow more health care workers to become vaccinated, extending the supply of the vaccine.

“However, since the vials are preservative free, it is critical to note that any further remaining product that does not constitute a full dose should not be pooled from multiple vials to create one,” the FDA said.

A Pfizer spokesperson said that each dose of the vaccine must contain .3 mL of the vaccine. The spokesperson said that the amount of vaccine in each vial varies, depending on what type of needles and syringes are being used.

According to FDA data, the federal government has allocated 2.9 million doses of the vaccine for the first round of distribution. Those being vaccinated will need to return in 21 days to get a booster in order for the vaccine to be fully effective.