https://www.nbcnews.com/health/health-news/how-effective-johnson-johnson-vaccine-what-know-n1259652
relevant text:
At first glance, the Johnson & Johnson vaccine looks to be the inferior option compared to the vaccines from Pfizer-BioNTech and Moderna, both of which reported around 95 percent effectiveness in their Phase 3 clinical trials.
Experts say that in no way should those percentages be interpreted in a way that suggests that the Johnson & Johnson shot — with its 72 percent effectiveness — is 23 percentage points less effective.
The percentages can't be compared directly to one another, because the trials were run differently. The vaccines were studied at different times during the pandemic, when different variants were circulating.