Differences among patients, not the genetic makeup of the coronavirus, determines how severe COVID-19 will be, a study finds.
Factors such as a person’s age and white blood cell counts are associated with disease severity, an analysis of 326 COVID-19 patients from Shanghai shows. Older people and people with low levels of certain immune cells known as T cells and high levels of an immune chemical called IL-6 tended to be sicker. But the version of the coronavirus that people were infected with made no difference in how sick they got, the team reports May 20 in Nature.
IL-6 is a protein known as a cytokine, one of many proteins that signal the immune system to rev up defenses. Overactive immune, known as cytokine storms, are a problem for people with severe cases of COVID-19.
In the new study, the team identified two major versions of the SARS-CoV-2 coronavirus, called clade I and clade II, by examining the genetic makeup of the virus from 94 of the cases and 221 genomes in the GISAID coronavirus database. GISAID is a repository that maintains thousands of viral genomes — the complete set of genetic instructions of a virus — compiled by researchers around the world. Those genomes are used in monitoring how the virus is evolving and tracing its path around the world.
Two mutations distinguish clade I from clade II. Other researchers had previously found the same genetic changes, and speculated that one version may be more virulent or transmit better among people. But the new data show no difference in contagiousness or disease severity between people infected with either clade.
Clade I was associated with six cases from the Huanan Seafood Wholesale Market in Wuhan. Clade II was found in early cases of the disease in Wuhan that weren’t associated with the market. Comparing the genetic makeup of the two versions, researchers conclude that the coronavirus probably made the leap from an animal to humans sometime in late November. The seafood market wasn’t where the virus originated, the team says, but was where people became infected with clade I, drawing attention to the new coronavirus.