The European Union's drug regulator announced on Monday that it was conducting an "accelerated" study into the possible need for a booster shot of the BioNTech-Pfizer jab.
It comes less than a week after the European Medicines Agency (EMA) said it saw no reason for a third dose of COVID-19 vaccines.
The EMA said it expects the results of its study to be ready "within the next few weeks." About 300 adults are participating in the study, all of whom are healthy adults who received their second dose of the vaccine about six months ago.
On Friday, vaccine maker Moderna had applied to the EMA to approve a third dose of its shot, a move Pfizer has already made.
The EMA is conducting a separate study using a third shot of both the BioNTech-Pfizer and Moderna vaccines to see if it increases antibody counts among the immunocompromised and very elderly.
Announcing the study, the EMA stressed that national health agencies of each individual EU member country may decide on their own to recommend a third dose.
What are the arguments around booster vaccines?
There has been some back-and-forth among experts about whether, and if so, when, a booster shot is necessary.
Some studies have shown antibody numbers decreasing over time and thus offering less protection, while other analysts have questioned whether pharmaceutical companies are trying to push their profits by convincing governments to buy third doses.
The World Health Organization has urged rich countries to wait on booster doses until at least the end of September. It says vaccines are more needed in developing countries, which have received fewer than 2% of the more than 5 billion doses administered.
Israel has already started administering booster doses. The US is gearing up to follow suit.
es/rt (AP, dpa)