But other countries are also signaling their intention to nationalize their approaches. The most promising clinical trial in China is financed by the government. And in India, the chief executive of the Serum Institute of India — the world’s largest producer of vaccine doses — said that most of its vaccine “would have to go to our countrymen before it goes abroad.”

George Q. Daley, the dean of Harvard Medical School, said thinking in country-by-country rather than global terms would be foolhardy since it “would involve squandering the early doses of vaccine on a large number of individuals at low risk, rather than covering as many high-risk individuals globally” — health care workers and older adults — “to stop the spread” around the world.

Given the proliferation of vaccine projects, the best outcome may be none of them emerging as a clear winner.

“Let’s say we get one vaccine quickly but we can only get two million doses of it at the end of next year,” said Anita Zaidi, who directs the Bill and Melinda Gates Foundation’s vaccine development program. “And another vaccine, just as effective, comes three months later but we can make a billion doses. Who won that race?”