Spain is extending the gap between the first and second doses of AstraZeneca's Covid-19 vaccine to 16 weeks for people aged under 60, the government said on Friday.
The country is the first to promote "off-label use" and to diverge from the European Medicines Agency's (EMA) usage authorisation.
The move is not approved by the regulator and it would make Spain responsible for any possible side effects.
The country initially gave AstraZeneca shots to essential workers aged 18 to 65 before allowing only those over the age of 60 to received the vaccine due to concerns about blood clots in younger people.
The EMA's approval for the vaccine is based on the second dose being administered between four and 12 weeks after the first. A 16-week interval has not been tested in any human trials.
AstraZeneca and the EMA did not immediately respond to requests for comment.
Spain's move to restrict use of the shot caused widespread uncertainty and meant some younger people who had already received a first dose were unable to receive a second.
By extending the interval between doses, authorities will be able to evaluate the results of mixing different vaccines before deciding whether those groups will receive a second AstraZeneca shot or another drug, the ministry said.
Spain's state-backed Carlos III Health Institute is currently investigating the effects of giving the Pfizer/BioNTech vaccine to patients who have already received an AstraZeneca shot.
The country's two-week coronavirus contagion rate fell slightly on Friday to 229 cases per 100,000 people, compared with 230 on Thursday, according to health ministry data.
The cumulative number of cases rose to 3,524,077, while the death toll rose to 78,216.
More on AstraZeneca
EU launches legal action against AstraZeneca over vaccine delays
Saudi Arabia denies reports of blood clots from AstraZeneca vaccine
One in a million: the truth about blood clots and Covid-19 vaccines