Last month, Merck began an early-stage trial of its other experimental Covid-19 vaccine. This one contains a weakened version of the virus that causes measles, and aims to deliver coronavirus's spike protein to the immune system, helping trigger an immune response.

Merck's approach wins the backing of some acclaimed scientists who note the company is using proven technology that can be scaled up readily for manufacture.

"They've been there, they've done it," says William Haseltine, a pioneering scientist of cancer and HIV drugs, and human genome projects.

Write to Gregory Zuckerman at gregory.zuckerman@wsj.com and GJared S. Hopkins at jared.hopkins@wsj.com

(END) Dow Jones Newswires

10-23-20 1248ET