More than half of patients hospitalized with suspected COVID-19 in Michigan during the state’s peak months received antibiotics soon after they arrive, just in case they had a bacterial infection in addition to the virus, a new study shows. But testing soon showed that 96.5% of them only had the coronavirus, which antibiotics don’t affect.

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The 3.5% of patients who arrived at the hospital with both kinds of infection were more likely to die. But the study suggests that faster testing and understanding of infection risk factors could help hospital teams figure out who those patients are – and spare the rest of their COVID-19 patients the risks that come with overuse of antibiotics.

The new paper, published in Clinical Infectious Diseases by a team from the University of Michigan, VA Ann Arbor Healthcare System and St. Joseph Mercy Health Care System, is based on data from more than 1,700 hospitalized patients.