In just six months, nearly 8 million people worldwide have been stricken with confirmed cases of Covid-19, and at least 434,000 have died. But those deaths have not been distributed evenly; among the most vulnerable are people with underlying health conditions, such as diabetes and diseases that affect the heart and lungs. According to a new modeling study, roughly 1.7 billion people around the world — 22 percent of the global population — fall into that category. Katherine J. Wu writes in the New York Times that that estimate, published in The Lancet Global Health, excluded healthy older individuals without underlying health conditions, a group also known to be at risk because of their age. It also did not take into account risk factors like poverty and obesity, which can influence a person’s susceptibility to disease and access to treatment.