The US Food and Drug Administration said on Wednesday that extra doses from vials of Pfizer Inc's COVID-19 vaccine can be used after reports of vaccine doses being thrown away by pharmacists due to labeling confusion.

Stat News reported earlier that hospital pharmacists found themselves in the position of throwing away one in every six doses of the first Pfizer-BioNTech COVID-19 vaccines distributed this week in the United States because of the confusion over labeling.

The Pfizer vials are supposed to hold five doses, according to the labeling, but media reports said pharmacists had found a way for a sixth or even a seventh dose. Without clear approval from the manufacturer, the extra dose had to be discarded.

"At this time, given the public health emergency, FDA is advising that it is acceptable to use every full dose obtainable (the sixth, or possibly even a seventh) from each vial, pending resolution of the issue," an FDA representative said in an emailed statement.

"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 statement added.

Rollout of the first tranche of 2.9 million doses of the newly authorized vaccine from Pfizer and German partner BioNTech SE continued for a third full day, with shipments headed to 66 more distribution hubs across the United States.

A second vaccine from Moderna Inc could win emergency-use approval from the FDA this week.

An additional 2 million doses of the Pfizer vaccine and 5.9 million doses of the Moderna vaccine could be allocated next week, US Health and Human Services Secretary Alex Azar said on a conference call on Wednesday. Two doses of the vaccines, given three or four weeks apart, would be required for each person being inoculated.