Pfizer has announced that those who have taken their vaccination will probably require a third dose. To date it is administering two doses of its vaccine.
CEO Albert Bourla said that if it is given, this will probably need to be administered from within six months to one year.
He was quoted as saying that after the third dose, the vaccine might need to be administered every year.
Researchers still do not know how long the vaccines keep protecting people after being administered.
In a study published by Pfizer itself, it concluded that its vaccine is 91% effective to protect the person from contracting the virus and 95% effective against severe illness up to six months after the second dose.
However, researchers have made it clear that more data is needed to establish whether the vaccine is effective after six months.
Meanwhile, Bourla did not confirm whether the price of the vaccine went up from €12 to €19.50 as claimed by Bulgarian PM Boyko Borissov.