In the early days of the COVID-19 pandemic, tens of thousands of young volunteers offered to risk their health by letting scientists intentionally infect them with the pandemic coronavirus, hoping to speed the hunt for a vaccine or treatment. Several research groups announced plans to run these so-called human challenge trials, even as some scientists questioned whether they could be conducted ethically. Now, with recent news that conventional human trials have produced several promising vaccines, scientists are debating whether planned challenge trials are still needed. In the United States, one nascent effort is on hold. In the United Kingdom, however, researchers say they are moving ahead. To read the full story.