France on Friday said AstraZeneca’s Covid-19 vaccine should only be given to people aged 55 and older because of concerns over blood clots.
The recommendation follows the EU medical regulator's assurance on Thursday that the shot was "safe and effective" after more than a dozen European countries suspended the vaccine over a possible link to blood clots.
France, Italy, Spain and Germany said within hours of the decision on Thursday that they would resume inoculations with the vaccine.
But France on Friday said AstraZeneca’s vaccine should not be used for people aged under 55.
The EU regulator's investigation also fell flat in the Nordic region.
Sweden, Denmark and Norway said they would wait until their own investigations are complete before lifting the suspension.
Finland said on Friday it would halt the use of AstraZeneca’s vaccine for one week to investigate two cases of blood clots in the brain that surfaced in recent days.
France's health authority said its decision was based on reports of blood clots that occurred in those aged under 55.
The surprise change is another twist in AstraZeneca's use in France – from an initial ban for over-65s, to no age limits, to a full suspension and now another age restriction.
Dominique Le Guludec, head of the French HAS health regulator, said such cases in those who had received the vaccine were very rare but also serious.
She said that while waiting for additional information, those under 55 should be vaccinated with the three other vaccines approved in France, from Pfizer-BioNTech, Moderna and Johnson & Johnson.
German scientists may have identified the cause of unusual clotting linked to the AstraZeneca vaccine, the Norddeutscher Rundfunk (NDR) public broadcaster reported.
Research by the Greifswald teaching hospital in northern Germany discovered that the vaccine caused rare thrombosis in the brain in a small number of patients. The condition can be treated with a common medication.
Officials from the European Medicines Agency said on Thursday that they could not definitively rule out a link between rare blood clot incidents and the vaccine.
But the watchdog said it was clear that the "benefits outweigh the risks" when using the drug made by AstraZeneca and Oxford University.
"The committee has come to a clear scientific solution: this is a safe and effective vaccine," the European agency's executive director Emer Cooke said.
"Its benefits of protecting people with Covid-19 outweigh the possible risks. If it were me, I would be vaccinated tomorrow.”
In France, out of 1.4 million AstraZeneca doses administered, blood clot cases were seen in only two people – a man, 51, and a woman, 24.
While European governments previously said they were ready to resume use once they received positive guidance, the suspensions may have caused irreparable damage to the reputation of the vaccine.
Questions were raised about the reliability of AstraZeneca's drug when some European countries chose to deny over-65s access to the shot, citing a lack of safety data, although those decisions were reversed.
Meanwhile, French Prime Minister Jean Castex received AstraZeneca’s Covid-19 vaccine on live TV on Friday as part of efforts to restore public confidence in the shot.
Mr Castex, 55, received the shot at the Begin military hospital in Saint-Mande, near Paris.
