Perhaps more importantly, the Johnson & Johnson shot is arguably more thoroughly tested than the (very thoroughly vetted) Moderna and Pfizer shots—owing to an accident of timing.
Each of the three companies went through rigorous, large-scale testing before the FDA gave them the nod for distribution. But Johnson & Johnson happened to test its vaccine at a time when new, more dangerous mutations or variants of the basic SARS-CoV-2 virus—“lineages” is the scientific term—were more prevalent.
That gives experts greater confidence in the new vaccine’s ability to protect against those lineages. Johnson & Johnson’s shot might appear less effective on paper. But that’s only because we have fresher data on it.
The gap between the Johnson and Johnson vaccine’s actual benefits and perceptions of those benefits isn’t hard to explain. Measuring the “effectiveness” of a vaccine is complicated. Explaining the results is even more complicated.