The antiviral drug remdesivir has been one of the few treatments that's seemed to help curb death rates in COVID-19 patients so ill that they need a ventilator.

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But a new study finds the drug might not be as effective for 'moderate' cases of COVID-19—patients hospitalized and perhaps needing oxygen to breathe, but not requiring a ventilator.

The study of nearly 600 such patients found that those who got a five-day course of remdesivir had a "statistically significant difference in clinical status compared to standard care, but the difference was of uncertain clinical importance," according to researchers led by Dr. Diana Brainard of Gilead Sciences, which makes remdesivir.