Presumably because they were less careful about avoiding exposure once they were fully vaccinated.
If a vaccine is less effective against some variants of SARS-CoV-2, I would expect that it does still offer protection against the most serious disease outcomes.
Here’s the Moderna timeline with links to press releases (easy for anyone to read, but releases include some general biotech info).
https://www.modernatx.com/modernas-work-potential-vaccine-against-covid-19
More info... First person dosed on March 16, 2020.
NIH Clinical Trial of Investigational Vaccine for COVID-19 Begins
Study Enrolling Seattle-Based Healthy Adult Volunteers
March 16, 2020
niaid.nih.gov/news-events/nih-clinical-trial-investigational-vaccine-covid-19-begins
Work began on a booster for all key variants including the South African variant (only in case needed). Further down in the article, the March 16, 2020 first vaccine recipient is mentioned again.
Moderna Announces First Participants Dosed in Study Evaluating COVID-19 Booster Vaccine Candidates
March 10, 2021 at 4:09 PM EST
https://investors.modernatx.com/news-releases/news-release-details/moderna-announces-first-participants-dosed-study-evaluating