"What Slaoui calls his “aha moment” came in the fall of 2019, when Moderna reported the results of a phase one clinical trial for a vaccine against cytomegalovirus, a common virus that can cause severe problems for people with weakened immune systems. After the phase one trial, he knew mRNA would work."

https://www.vanityfair.com/news/2023/10/operation-warp-speed-covid-19-vaccine