already high in Europe, especially toward the AstraZeneca shot, on which Europe has built its plans. The proportion of people willing to get the shot has, in some polls, dropped significantly below the 70 percent needed to achieve herd immunity.

“High-profile, vivid events that are really scary have a way of controlling the public imagination,” Dr. Faden said.

Johnson & Johnson with a lower efficacy rate than two-shot variants, health officials hailed its simpler distribution as a breakthrough in the push for herd immunity. Americans have largely gone along.

“We don’t always see it,” Dr. Persad said of these ethical trade-offs, “but it actually comes up all the time.”

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