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.

That estimate, published today 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…

The study also estimated that about 4 percent of the world’s population, around 349 million people, would require hospitalization if they became infected. That number includes patients without underlying medical conditions, such as healthy, older adults; and the risk of hospitalization increases with age.