Covid-19 pneumonia generally develops between five and 10 days after infection. It does not cause shortness of breath in most patients. Oxygen levels drop over days, and patients gradually increase their respiratory rate. The low oxygen saturation happens silently — silent hypoxia, we call it — and patients do not realize it. By the time patients feel shortness of breath or have evident trouble breathing and head to the hospital, they already have alarmingly low oxygen saturations.

A majority of Covid-19 pneumonia patients that we treated or observed during the surges in New York City and Italy had severe lung injury on first presentation. They were, in other words, arriving at the hospital too late, and many were winding up on ventilators.

In all medicine — whether in patients who have traumatic injury, cancer, diabetes or an infectious disease — earlier identification and treatment leads to better outcomes. Covid-19 is no different. We must continue improving the I.C.U. care of patients with advanced Covid-19 pneumonia. But we will have the greatest public health impact if we prevent it from occurring in the first place.