Medicine has long benefited from robust debate among practitioners, scientists, and scholars, but the response of the United States and other nations to the COVID-19 pandemic has left some highly acclaimed doctors believing their field is in the grip of dangerous groupthink.

White House COVID-19 czar Dr. Anthony Fauci and former National Institutes of Health Director Francis Collins have aggressively silenced doctors who don’t toe the government line on matters including vaccine mandates, natural immunity, and the safety and efficacy of inoculations. But the questions haven’t gone away, and doctors who have been asking them at their own professional peril haven’t, either. Below are the stories of four esteemed physicians who have challenged the COVID-19 status quo and the medical establishment they know well.