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Pneumonia is an infection of the air sacs in the lungs, typically caused by a bug, bacteria, or influenza and now, SARS-CoV-2. But doctors say the virus leaves more severe damage in its wake. And now they believe they know why. It is knowledge that will lead to better treatments.

Dr Benjamin Singer knows pneumonia. He’s been studying it for years. But with COVID-19, it’s different.

“I think it’s not widely understood that at its heart severe COVID is a pneumonia,” he said. “What we found is the immune system responds to the pneumonia caused by COVID-19 in a different way than it responds to patients with pneumonia caused by influenza or bacteria.”

The Northwestern Medicine pulmonologist and critical care doctor and his colleagues collected fluid samples from 86 COVID-19 patients on ventilators. They compared the samples to 256 ventilated patients with other types of pneumonia. Instead of rapidly infecting large regions of the lung like a more typical pneumonia, SARS-CoV-2, the virus that causes COVID-19, takes hold in multiple small areas of the organ — infecting normal immune cells. 

“So the virus we found actually infects some of the normal immune cells in the lungs called macrophages,” Singer said. “And this is an unusual thing because when this virus infects those immune cells, those immune cells become very inflammatory and they can lead to damage themselves.”

Singer said think of it like a wildfire.

“What we see in COVID is more like the wildfires in the West where you have little embers that kind of pop up then die out and spread to other areas and ignite over time,” he said.

And time is a major factor.

“It may not be the severity of the inflammation at any one point, but because this disease lingers for so long, over time you are exposed to that inflammation over a much longer period of time than you might be with that kind of hot fire that burns very quickly then dies out quickly,” Singer said. “The disease itself won’t go away. It’s going to be with us for some time. And so finding better ways to help people is important thing to do.”

And that is the next step is testing drugs already in existence to see if they can tamp down the inflammatory response in immune cells. Northwestern hopes to start a clinical trial in the coming weeks.