U.K. says under-40s should be offered an alternative to AstraZeneca’s vaccine.

Britain’s vaccines regulator advised on Friday that all adults under 40 in the country should be offered alternatives to AstraZeneca’s Covid-19 vaccine. It factored in concerns over very rare blood clots, the dwindling risk of severe coronavirus infection in younger adults and the availability of alternatives.

The guidance extends earlier advice that people under 30 would be offered alternative doses.

The use of the AstraZeneca vaccine has been marred by uncertainty after reports of a possible link between the doses and very rare blood clots, but public health experts around the world say that the vaccine’s benefits far outweigh the risks for most people.

Britain’s Joint Committee on Vaccination and Immunization stressed that the chances of younger people becoming seriously ill with the coronavirus had grown smaller as infection rates decrease across the country. It said that this new reality paired with the availability of alternative vaccines had factored into the decision.

But the country is also closely monitoring new variants of the coronavirus, and on Friday public health officials in England noted that a variant first detected in India was now considered a “variant of concern” — meaning that it is at least as transmissible as the dominant variant in Britain. The cases identified in the country more than doubled from 202 to 520 in the week, but still account for just a fraction of the cases there.

While there is still not enough evidence to indicate whether any of the variants recently detected in India cause more severe illness or render vaccines any less effective, Britain is proceeding with caution. Most of the identified cases of the variant are in London and in the town of Bolton in the northwestern England.

Regarding the AstraZeneca vaccine, the regulator advised that where available, an alternative should be offered to healthy adults under 40, though it stressed that potentially severe side effects from the doses were “extremely rare.” It noted that “for the vast majority of people, the benefits of preventing serious illness and death far outweigh any risks.”

The vaccination committee advised that anyone who received a first dose of the AstraZeneca vaccine should still receive a second, except those who experienced clotting.

Britain’s medicines regulatory agency had received reports of 242 cases of blood clots accompanied by low platelet counts in people who received the AstraZeneca vaccine through April 28. As of that date, about 22.6 million AstraZeneca doses had been administered in Britain, including about 5.9 million second doses.

Over all, about 35 million people in the country have received at least one dose of a coronavirus vaccine.