Health-care companies ticked down as investors hedged their bets on the outcome of the Covid-19 race.

AstraZeneca shares fell slightly after the British drug maker and the University of Oxford said their vaccine was as much as 90% effective in preventing the infection without serious side effects in large clinical trials, though they said the vaccine's efficacy varied widely based on dosage.

That was viewed by some investors as a less propitious outcome than the trial results logged by Pfizer -- and its partner, BioNTech -- and Moderna. Still, it's looking increasingly certain that vaccines for the coronavirus will soon become available.

The U.S., U.K. and Germany have announced plans to start inoculating some citizens in December, with Spain preparing a campaign for January, as reported earlier. Regeneron Pharmaceuticals' Covid-19 antibody drug was authorized Saturday by U.S. health regulators, the second cleared this month to treat patients who aren't hospitalized but are at high risk of developing severe disease.

Merck agreed to buy biopharmaceutical company OncoImmune, which has reported positive results from a late-stage study of a coronavirus therapeutic candidate, for $425 million in cash.

Write to Rob Curran at rob.curran@dowjones.com

(END) Dow Jones Newswires

11-23-20 1722ET