A world eager for progress on vaccines to guard against the SARS-CoV-2 coronavirus and/or COVID-19 might have to wait a bit longer for a breakthrough. The CEO of Moderna (MRNA 2.70%), the developer of what many experts consider the leading vaccine candidate, said Friday that his company is slowing enrollment in its crucial phase 3 clinical trial.

The reason is that Moderna wants more individuals from minority groups to participate in the study, said Moderna CEO Stephane Bancel in an interview with CNBC. "We want to ensure we have data for all the people who could benefit and be protected," he said.

Child getting a jab from a medical professional.

Image source: Getty Images.

Research shows that, due to a variety of societal and medical factors, minorities are more at risk of contracting and dying from COVID-19. But it isn't proving easy for the company to get people from those communities to sign up for the research.

As of Aug. 28, Moderna had recruited 17,458 individuals for its late-stage trial out of its goal of 30,000 total. Of those enrolled so far, 68% are white, 20% are classified Hispanic or Latino, while the black or African-American and Asian categories are in the single digits, at a respective 7% and 3%. The remaining 2% are "other or unknown." It wasn't immediately clear what its desired levels were for the minority groups.

Moderna's vaccine candidate, mRNA-1273, has been developed relatively quickly. It's one of only several vaccine candidates that has advanced to a phase 3 clinical trial.

On Friday, Moderna's shares sank by almost 3.5%, a steeper fall than that of the S&P 500 index on the day.