Moderna (NASDAQ:MRNA) is one step closer to getting its COVID-19 vaccine, mRNA-1273, on the market. The biotech completed enrollment of 300 older adults, 55 years and older, in a phase 2 clinical trial.

The other half of the clinical trial, 300 younger adults ages 18-55, was completed last month, but the older-adult cohort took longer to enroll because it was set up in two stages with a pause after 50 patients to observe safety.

In addition to the older adults in the phase 2 clinical trial, there's also a cohort of 30 older adults, ages 56-70, and one of 30 elderly adults, ages 71 and above, in the phase 1 study. Investors continue to wait for detailed data from that clinical trial.

Safety and efficacy in older and elderly adults is important since those populations are especially vulnerable if they get infected with the novel coronavirus that causes COVID-19.

An injection being given into a person's upper arm

Image source: Getty Images.

Phase 3 clinical trial progress

Over the last week, reports suggested that the start of Moderna's phase 3 clinical trial was delayed because of changes to the protocol for the study and because Moderna was butting heads with officials at the National Institutes of Health (NIH) and the Food and Drug Administration, as well as the NIH's outside medical experts.

But Moderna noted in the press release announcing the completion of enrollment in the phase 2 study that the phase 3 protocol is finalized, and Tal Zaks, chief medical officer at Moderna, said the phase 3 study "remains on track to start in July."