Gilead Sciences Inc.’s drug remdesivir showed only a limited benefit in a large trial, a result that may shift perceptions of the first therapy cleared for use in severe cases of COVID-19.

In the phase 3 trial, a group of moderately ill, hospitalized patients getting the drug for five days showed a modest improvement compared to those getting the standard of care, the company said in a statement.

But another group getting the drug for 10 days didn’t show a statistically significant improvement, which is likely to raise questions about why a longer course doesn’t help more.

The shares were down 1.7 per cent in trading before the markets opened in New York, after earlier falling as much as 6.4 per cent. Severely ill patients weren’t included in the trial.