Efficacy depends on the details of a trial, such as where it took place. Johnson & Johnson ran trials at three sites: in the United States, Latin America and South Africa. The overall efficacy was lower than that in the United States alone. One reason for that appears to be that the South Africa trial took place after a new variant had swept across that country. Called B.1.351, the variant has mutations that enable it to evade some of the antibodies produced by vaccination. The variant didn’t make the vaccine useless, however. Far from it: In South Africa, Johnson & Johnson’s efficacy was 64 percent.
Efficacy can also change when scientists look at different outcomes. Johnson & Johnson’s vaccine had an 85 percent efficacy rate against severe cases of Covid-19, for example. That’s important to know, because it means that the vaccine will prevent a lot of hospitalizations and deaths.
Author(s): Carl Zimmer, Keith Collins
Publication Date: 3 March 2021
Publication Site: New York Times