Interesting, I never bothered to investigate the basis of that claim. According to https://www.nature.com/articles/d41586-022-02328-0, a recent study does show that the vaccine was effective at preventing the spread, which is unsurprising to me. What is surprising is that the article claims this was the first study of the effectiveness against spreading an infection.
It would only be a measure of postponing some of the rising transmission rates if people don't get boosted every few months. Also it would have to be compared against the effects of natural immunity.
