Moderna Inc MRNA.O has been asking sites that are conducting clinical trials of its experimental coronavirus vaccine to focus on enrolling at-risk minorities, even if that slows down the trial speed, the company said on Friday.

Shares of Moderna, one of the few companies in the final stages of developing a safe and effective COVID-19 vaccine, closed down about 3.5%.

The company said it has enrolled 21,411 participants in the study so far. It had 17,000 participants as of last week, with 24% from communities of color.

The drug developer aims to recruit 30,000 healthy volunteers and said it expected enrollment in the late-stage study, which began in late-July, to be completed in September.

A growing body of evidence has shown that long-standing health and social inequities have resulted in increased risk of infection and death from COVID-19 among communities of color.

Nearly a fifth of 11,000 people enrolled so far in a 30,000-volunteer U.S. trial testing a COVID-19 vaccine from Pfizer and German partner BioNTech  are Black or Latino, groups among the hardest hit by the coronavirus, a top Pfizer executive told Reuters last month.