Merck is expanding further into the biotech sector by acquiring Rockville, Md., biopharma company OncoImmune for $425 million, netting in the process an investigational drug that has shown promise as a treatment for hospitalized COVID-19 patients.

As some of the world’s largest drugmakers gear up to manufacture and distribute COVID-19 vaccines, a relatively tiny competitor, Massachusetts-based Moderna, faces some unique challenges.