Much remains unknown about the path the virus takes in the sickest patients, but an increasing number of experts believe a hyperactive immune response, rather than the virus, is what ultimately kills many Covid-19 patients.

The out-of-control immune response eventually causes the patients’ lungs to stop delivering oxygen to the rest of organs, leading to respiratory failure and in some cases death, the experts say. The malfunctioning immune system may be driving the rapid decline in lung function experienced by some patients, including younger and relatively healthy ones, after the initial onset of symptoms, doctors say…

As scientists race to better understand the phenomena, pharmaceutical companies including Roche Holding AG are partnering with hospitals to explore whether drugs proven to tamp down an out-of-control immune response could help the sickest Covid-19 patients…

“You remove one piece of the storm, and it can quiet the whole thing,” said Kevin Tracey, president of the Feinstein Institutes for Medical Research at Northwell Health, which is testing Kevzara, an anti-inflammatory drug from Regeneron Pharmaceuticals Inc.