The drugs often prevent severe disease, keeping people like Mike Burton out of the hospital if taken within seven to 10 days after symptoms begin. And since last month, they can be given prophylactically to millions of people like Linda Burton who have been exposed to the coronavirus and are at high risk of serious consequences.

“That was all news [to me], when my friend Rita called,” said Linda Burton, a retired nurse. “I want everybody to know about this. I’m telling people that I know that are older. I’m saying, ‘If you get exposed, you need to talk to your doctor about it.’ ”

Monoclonal antibodies are free to patients and there have been almost no side effects. They are accessible on an outpatient basis, via a single infusion or four injections. Hospitals, urgent-care centers and even private doctors are authorized to dispense them.

But Regeneron Pharmaceuticals, maker of the only authorized, free monoclonal antibodies, said it is reaching fewer than 30 percent of eligible patients, up from fewer than 5 percent a month ago.