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Dr. Nicole Lurie, a former assistant secretary for preparedness and response at the U.S. Department of Health and Human Services and currently an adviser at the Coalition for Epidemic Preparedness Innovations, said the lesson that the U.S. government should draw from Mr. Putin’s announcement is clear.

“This is exactly the situation that Americans expect our government to avoid,” she said.

A faster process

Along with determining whether the vaccine protects people, Phase 3 trials can reveal uncommon side effects that may not have shown up in the comparatively small number of volunteers who enrolled in the earlier phases.

Just because someone gets sick or dies after getting a vaccine, however, doesn’t necessarily show that the vaccine was the culprit. By comparing large groups of people who received the vaccine versus the placebo, researchers can identify unusual clusters of cases in the vaccinated participants.

Along the way, vaccine developers share these results in reports to government regulators and in peer-reviewed papers for scientific journals. Outside experts then evaluate the data from Phase 3 trials and give their recommendation to the F.D.A., which then decides whether to approve a vaccine for widespread use.

“It’s not enough for me to say I have a great product,” said Dr. Salmon. “Before you use it, you need other people to really look at the data and be convinced that the benefits outweigh the risks.”

And even after a vaccine is licensed, researchers still keep an eye on it to make sure it’s safe. As millions of people get a vaccine, even rarer side effects may emerge over time. It’s also possible that certain groups of people, such as children or the elderly, turn out to face risks from a vaccine that weren’t immediately clear from the Phase 3 trials.

Regulators can then make adjustments to the vaccine — changing the dose, for example — to make it safer.