SARS-CoV-2 is the virus. COVID-19 is the disease that it causes. The two aren’t the same. The disease arises from a combination of the virus and the person it infects, and the society that person belongs to. Some people who become infected never show any symptoms; others become so ill that they need ventilators. Early Chinese data suggested that severe and fatal illness occurs mostly in the elderly, but in the U.S. (and especially in the South), many middle-aged adults have been hospitalized, perhaps because they are more likely to have other chronic illnesses. The virus might vary little around the world, but the disease varies a lot.
This explains why some of the most important stats about the coronavirus have been hard to pin down. Estimates of its case-fatality rate (CFR)—the proportion of diagnosed people who die—have ranged from 0.1 to 15 percent. It’s frustrating to not have a firm number, but also unrealistic to expect one. “Folks are talking about CFR as this unchangeable quantity, and that is not how it works,” says Maia Majumder, an epidemiologist at Harvard Medical School and Boston Children’s Hospital.
The CFR’s denominator—total cases—depends on how thoroughly a country tests its population. Its numerator—total deaths—depends on the spread of ages within that population, the prevalence of preexisting illnesses, how far people live from hospitals, and how well staffed or well equipped those hospitals are. These factors vary among countries, states, and cities, and the CFR will, too. (Majumder and her colleagues are now building tools for predicting regional CFRs, so local leaders can determine which regions are most vulnerable.)
The variability of COVID-19 is also perplexing doctors. The disease seems to wreak havoc not only on lungs and airways, but also on hearts, blood vessels, kidneys, guts, and nervous systems. It’s not clear if the virus is directly attacking these organs, if the damage stems from a bodywide overreaction of the immune system, if other organs are suffering from the side effects of treatments, or if they are failing due to prolonged stays on ventilators.