“Safety has been our priority throughout the development of the vaccine,” Andrew Pollard, the Oxford researcher in charge of the trials, said on Wednesday. The identification of the clots, he added, “shows that the safety system works.”

In the United States, AstraZeneca is preparing to apply for emergency use authorization from the Food and Drug Administration. If and when they take up the application, that agency’s regulators are expected to scrutinize the clotting cases.

The United States, flush with vaccines from three other makers, may not ultimately need AstraZeneca’s shot. But any ruling by the F.D.A. is expected to hold considerable weight in some of the poorer nations that are relying on the shot.

The World Health Organization said a vaccine safety subcommittee had met on Wednesday and noted that “rare adverse events following immunizations should be assessed against the risk of deaths from Covid-19 disease and the potential of the vaccines to prevent infections.” It said that a link with the clotting problems, while “plausible,” had not been confirmed.

For Britain, the AstraZeneca vaccine has become a huge source of national pride, and the backbone of the country’s speedy inoculation program.

Even if younger people are at lower risk from severe Covid-19, scientists have said, inoculating them remains essential to creating enough protection in the population to end the pandemic.

Emma Bubola, Monika Pronczuk and Rebecca Robbins contributed reporting.

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