Why Tens Of Thousands Of People Are Key To Testing A COVID-19 Vaccine

Researchers in Miami hold syringes containing either a placebo or the candidate COVID-19 vaccine from Moderna. Their work is part of a phase three clinical trial sponsored by the National Institutes of Health.

Taimy Alvarez / AP

Updated at 7:00 a.m. ET

More than 100,000 people are taking part in studies to see if one or more COVID-19 vaccine candidates actually work.

Janssen Pharmaceutical Cos. of Johnson & Johnson today starts wide-scale testing for its vaccine. It will involve as many as 60,000 volunteers worldwide. AstraZeneca, Pfizer and Moderna already have vaccine candidates in large studies in the United States. Novavax should start its study later this fall.