The Danish Health Authority confirmed its updated guidelines in a statement.
Up to 7 vaccine doses can be drawn from Pfizer vials, while those from Moderna contain a maximum of 12.
In some cases, it is not possible to draw the maximum number of doses. In such events, leftover vaccine can be combined with remainders from another vial, the health authority said.
No more than two vials may be combined and they must come from the same supplier and batch.
“We must continue at vaccination centres to strive to withdraw as many full doses of vaccine from each multi dose vial as possible,” Danish Health Authority department chief Bolette Søborg said in the statement.
“When it occasionally is not possible to withdraw all of the recommended full doses from the vials, the leftover can be combined with vaccine from one other vial to make a full dose,” she explained.
Experienced medical personnel at vaccine centres will be responsible for putting doses together.
The Danish Health Authority and Danish Medicines Agency were both previously opposed to mixing vials, according to an official note relating to the decision.
The health authority was concerned over the availability of qualified staff to mix the vaccines, which could lead to contamination or incorrect combination, for example using different batches.
But regional health authorities have reportedly developed procedures aimed at preventing this, while vaccine centres have also become more efficient with increased experience, according to the health authority.