Pfizer (NYSE:PFE) has now enrolled 36,576 participants in the late-stage clinical trial of its coronavirus vaccine candidate, BNT162b2, which it's developing with BioNTech (NASDAQ:BNTX). Last month, the companies increased their planned enrollment in the study from 30,000 to approximately 44,000, meaning that it is currently 83% enrolled.

Pfizer and BioNTech have signed up more participants than Moderna (NASDAQ:MRNA), which had 28,043 people in the clinical trial of its vaccine candidate, mRNA-1273, as of Friday evening. Moderna's phase 3 trial still has a target of 30,000 participants, so it'll likely reach full enrollment first. But the amount of time it will take the companies to reach complete enrollment shouldn't much affect when they release their initial efficacy data, because those preliminary results will come from analyses of data generated by the participants inoculated earliest in the studies.

Researchers in these trials can't begin to measure their candidates' efficacy -- i.e., how well a vaccine protects against COVID-19 compared to a placebo -- in participants until after they've gotten their second doses of the two-dose regimens. Moderna has given its booster shots to 19,369 participants, while 28,146 participants in Pfizer and BioNTech's study have received their second doses. Therefore, it seems likely Pfizer and BioNTech will be able to make an initial data readout first.

Gloved hands giving an injection into a shoulder

Image source: Getty Images.

Diversity in the clinical trials has recently become a hot-button issue in the industry. It was cited by Pfizer and BioNTech as one of the reasons they up-sized their study. And Moderna's CEO said he wanted more diversity in its study even if that resulted in slower enrollment.

On that front, Moderna is making better headway in the U.S.: 33% of its trial participants are from diverse communities, compared to Pfizer and BioNTech's 28%. Moderna's study is entirely in the U.S., but Pfizer and BioNTech are also enrolling participants elsewhere, which has helped increase the diversity in the overall study population to 42%, mostly due to the addition of participants with Hispanic backgrounds.