I'm way out of my depth on this topic, but my understanding is that studies have shown that several currently approved mRNA vaccines cause a similar reduction in these cancers. Moderna's goal with this trial was to show their tailored vaccine outperformed a COVID vaccine (for example), and my assumption from the stock dip is that smarter people than I were not convinced.
https://www.nature.com/articles/s41586-025-09655-y