It’s a controversial idea: Intentionally infect people with the virus that causes Covid-19 to test the effectiveness of a potential vaccine.

The approach is called a human challenge trial, and it’s not the usual way a vaccine is tested. More commonly, researchers track thousands of people, some of whom receive a vaccine, and others a placebo, and then see who becomes infected in the natural course of their lives. It’s a slower process, but poses fewer risks than deliberately infecting people after they’ve received a vaccine.

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