18 cases. I'm not sure I understand your point about breakthrough infections in other groups; do you mean that a significant percentage of vaccinated individuals are getting infected, compared to unvaccinated individuals?
Enough vaccinated individuals are being infected for the CDC to conclude in the Provincetown study that "the vaccinated may spread the Delta variant as easily as the unvaccinated." Any time a breakthrough case is reported now you can consistently find statements from public health officials saying "the vaccines don't necessarily keep you from getting sick but they are still effective against hospitalization and death." Given that they don't do routine testing of trial subjects, we can't really even conclude anything from "18 cases in the control group and none in the experimental" except that the vaccines are suppressing symptoms of illness.
Second the other child comment's request for a source on "18 cases," the only thing I can find in the Pfizer press release and major media sources on this is statements about antibody levels.
https://www.pfizer.com/news/press-release/press-release-deta...