Hydroxychloroquine as Postexposure Prophylaxis for Covid-19

To the Editor: The window for postexposure prophylaxis against Covid-19 is narrow. 1-3 Therapy that is initiated up to 4 days after exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is early treatment, not postexposure prophylaxis. The trial described in the article by Boulware et al. (published online on June 3 at NEJM.org) 4 was therefore largely about the prevention of symptoms in persons who may already have been infected. The trial was designed to detect a 50% relative reduction in new cases of symptomatic Covid-19; this estimate was overly optimistic. The trial was not powered to detect an important, but . . .