Prime Minister Boris Johnson had been expected to trumpet a rare success in the campaign against the coronavirus Friday: news that Britain had vaccinated 5.4 million people. By the end of the day, it was overtaken by a tentative finding that a new variant of the virus may be deadlier than the original.

That possibility, raised by preliminary studies relying on small numbers of deaths in hard-hit hospitals, remains far from conclusive. But the prospect that the fast-spreading new variant, already known to be more contagious, could also be more lethal compounded fears that even with the arrival of vaccines, the pandemic will remain a severe threat for some time.

Government scientists said the early evidence suggests that the new variant, first detected late last year in Britain, could raise the risk of death by some 30%. But even with such an increase, the great majority of cases are not fatal, and the government estimates included a broad range of possible effects.