But the effects on the immune system are what concern some researchers the most. Obesity can cause chronic, low-grade inflammation, which is thought to contribute to the increased risk of conditions such as diabetes and heart disease2. As a result, people who are obese might have higher levels of a variety of immune-regulating proteins, including cytokines. The immune responses unleashed by cytokines can damage healthy tissue in some cases of severe COVID-19, says Milena Sokolowska, who studies immunology and respiratory diseases at the University of Zurich in Switzerland. And the constant state of immune stimulation can, paradoxically, weaken some immune responses, including those launched by T cells, which can directly kill infected cells. “I would say they are more exhausted at the start in their fight with infection,” says Sokolowska.

Preliminary evidence suggests that SARS-CoV-2 infections linger for about five days longer in people who are obese than in those who are lean, says endocrinologist Daniel Drucker of the Mount Sinai Hospital in Toronto, Canada. “That would imply that these people are having trouble clearing the infection,” he says. “They may have trouble mounting normal viral defences.”