Johnson & Johnson cuts size of Covid-19 vaccine study due to prevalence of disease in U.S.

Johnson & Johnson is cutting the size of its pivotal U.S. Covid-19 vaccine trial — the only major study testing a single dose of a Covid vaccine — from 60,000 volunteers to 40,000 volunteers.

The change is being made possible by the fact that Covid-19 is so pervasive across the country, according to a person familiar with the matter. The more virus there is in the U.S., the more likely it is that participants will be exposed to it, meaning researchers will be able to reach conclusions based on a smaller trial.