In the last week, Moderna (MRNA -1.75%) substantially slowed enrollment in the phase 3 clinical trial testing its coronavirus vaccine, mRNA-1273. After a slight slowdown last week, the biotech took a major breather, enrolling just over half the number of participants it had enrolled in the previous week.

Date of Update Enrollment Week-Over-Week Enrollment
Aug. 28, 2020 17,458 4,264
Sept. 4, 2020 21,411 3,953
Sept. 11, 2020 23,497 2,086

Source: Moderna.

The biotech said the reason for the slower enrollment is its commitment to increase diversity in the clinical trial. The fraction of white participants being enrolled in the study has decreased over the last four weeks, from 73% to 68% to 67%, and finally down to 59% for the most recent week. Meanwhile, enrollment of Hispanic or Latino participants jumped from 17% to 22%, and Black or African Americans increased from 5% of enrollment three weeks ago to 11% of enrollment in the most recent week.

Gloved hands giving an injection into an arm

Image source: Getty Images.

Moderna is trailing Pfizer (PFE -1.82%) and BioNTech (BNTX -0.61%), which had enrolled 25,189 patients in the phase 2/3 clinical trial testing its coronavirus vaccine, BNT162b2, as of an update on Sep. 7. In terms of diversity though, Moderna is ahead with 27% of participants coming from diverse communities, compared to Pfizer and BioNTech's 24%.

Fortunately for Moderna, the last few participants enrolled in the 30,000-patient study don't have much of an effect on the initial efficacy readout. Participants being enrolled now will get their booster shot four weeks from now and will start contributing to the efficacy readout -- whether they get COVID-19 -- two weeks after that. By comparison, patients enrolled early in the study, which started in late July, are starting to contribute to the efficacy readout now.