When President Trump promoted an experimental drug as a “cure” for Covid-19 in a video on Wednesday, it might have seemed that he was at it again: touting a questionable fix for a deadly pandemic, not unlike his earlier enthusiasm for the malaria drug hydroxychloroquine or even, at one point, disinfectant.

But the treatment that Mr. Trump extolled, which was administered last week after doctors diagnosed Covid-19, is not a fringe product. It’s a promising drug in the final stages of testing developed by a respected biotech company, Regeneron. Infectious disease experts have been closely following the treatment, as well as a similar product from Eli Lilly, in the hopes that the therapies could be a real advance in the fight against Covid-19.

Pharmaceutical companies often pay handsomely for celebrity endorsements, but this patient testimonial was like no other. It came from a polarizing president who, just weeks away from