They are comparable in terms of their and . One important difference is that the efficacy of the AstraZeneca vaccine was assessed after two doses while that of Johnson & Johnson was assessed after only one dose. A single dose of the Johnson & Johnson vaccine offers a degree of protection that is comparable to two doses of the AstraZeneca vaccine. Preliminary study results show that a second vaccination with Johnson & Johnson's vaccine can further increase its efficacy.
Additionally, the two vaccines use different viral vectors, or vehicles, by which they transfer the construction plan for the spike protein. In the case of the AstraZeneca vaccine, a harmless adenovirus found in chimpanzees is used, while the Johnson & Johnson vaccine uses an attenuated (and harmless) human adenovirus.