Although it seems unlikely that the United States will shift course, its neighbor to the north has embraced a delayed strategy to cope with a booming pandemic and a short supply of vaccines.

Dr. Catherine Hankins, a public health specialist at McGill University in Montreal and a member of Canada’s Covid-19 Immunity Task Force, endorsed that decision, based on the emerging evidence about single doses. And she said she thought that other countries facing even worse shortfalls should consider it as well.

“I will be advocating at the global level that countries take a close look at Canada’s strategy and think seriously about it,” Dr. Haskins said.

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