Moderna has asked the U.S. Food and Drug Administration for permission to fill their Covid-19 vaccine bottles with up to five additional doses to help clear a manufacturing bottleneck, according to a person familiar with the matter.

The change would allow Moderna to fill 15 cans into vials of the same size, now cleared for 10, which eases the pressure on the manufacturing process known as filling / finishing, said the person who refused to named because the application is not public yet.

The availability of Covid-19 vaccines has caused frustration since their approval in the US in mid-December. While the pace of administration has increased to an average of more than 1 million a day, the limited supply has hampered states’ ability to operate mass vaccination centers. By Friday, the US had distributed 49.2 million doses and 27.9 million had been given, according to the Centers for Disease Control and Prevention.

“We have problems making these mRNA vaccines,” said Dr. Paul Offit, director of the Vaccine Education Center and a physician at Philadelphia Children’s Hospital. “We have up to 1.2 million doses a day when we need 3 million doses a day.”

The FDA declined to comment and asked questions to the company. Moderna did not immediately respond to a request for comment.

The move from Moderna came after Pfizer requested and received a change in emergency approval from the FDA to specify that the Covid-19 vaccine bottles contain six doses instead of five after pharmacists determined that it had a bonus dose the correct syringes could be extracted. Pfizer then said it would ship fewer vials to the US, but the same number of doses specified in its contracts.

Moderna vials have also been found to contain a bonus dose, but a policy change is being sought to add volume to the vials.

The bottleneck is not the vials themselves, but the manufacturing capacity to fill the vials. The manufacturing filling / finishing process must be performed under aseptic conditions to ensure contamination does not occur and the capacity is high.

Companies have begun to form manufacturing partnerships that focus on this step in the process to increase production. Novartis announced on Friday that it has signed an initial vial fill agreement for BioNTech, Pfizer’s partner in Europe, for the Covid-19 vaccine.

“We expect this to be the first in a series of such agreements,” said Steffen Lang, head of technical operations at Novartis.