Secondary bacterial pneumonia killed many COVID-19 patients who were on a ventilator, according to a new study.

Researchers from Northwestern University in Illinois looked at data from 585 mechanically-ventilated patients with severe pneumonia and respiratory failure. Of those patients, 190 had COVID-19. The patients were in the intensive care unit (ICU) at Northwestern Memorial Hospital.

A ventilator is a machine that is used to help a patient breathe by providing oxygen.

The authors developed and employed a machine learning approach called CarpeDiem to analyze the medical record data of the patients.

They found that nearly half of the ventilated COVID-19 patients in the study developed a secondary pneumonia caused by bacteria, after the patient had already developed an upper respiratory infection that was caused by the SARS-CoV-2 virus.

The condition, termed ventilator-associated pneumonia (VAP) in the study, was not associated with mortality overall.

But VAP that was not successfully treated was associated with greater mortality, the authors concluded. They found that the mortality was higher in those with VAP who were unsuccessfully treated compared to those who were successfully treated, at 76.4 percent mortality in the former group compared to 17.6 percent in the latter group.

“In all patients, including those with COVID-19, CarpeDiem demonstrated that unresolving VAP was associated with transitions to clinical states associated with higher mortality,” the authors reported.

“Those who were cured of their secondary pneumonia were likely to live, while those whose pneumonia did not resolve were more likely to die,” he added. “Our data suggested that the mortality related to the virus itself is relatively low, but other things that happen during the ICU stay, like secondary bacterial pneumonia, offset that.”

Singer also said that the COVID-19 patients in the study did not show any evidence of a so-called cytokine storm.

“The term ‘cytokine storm’ means an overwhelming inflammation that drives organ failure in your lungs, your kidneys, your brain and other organs,” Singer said.

“If that were true, if cytokine storm were underlying the long length of stay we see in patients with COVID-19, we would expect to see frequent transitions to states that are characterized by multi-organ failure. That’s not what we saw.”

Study co-author Dr. Richard Wunderink said in a statement that a secondary bacterial infection of the lungs as a contributor to deaths in COVID-19 patients “has been underappreciated because most centers have not looked for it,” or they only look at outcomes in terms of whether there was a secondary bacterial infection present and not whether treatment of it was successful.