Here is another interesting article specifically about the Moderna vaccine - mRNA-1273, which I suspect is going to be the prominant US option. Not positive.

Talks a lot about logistics, trials, ...
https://www.precisionvaccinations.com/vaccines/mrna-1273-sars-cov-2-vaccine

From the article ...
"As of September 16, 2020, there were 25,296 participants enrolled in the COVE Phase 3 clinical study. And as of October 2, 2020, 19,369 participants have received their 2nd vaccination of Moderna's mRNA-1273 vaccine candidate"