DURBAN, South Africa — Doctors and nurses at a South African hospital group noticed an odd spike in the number of Covid-19 patients in their wards in late October. The government had slackened its lockdown grip, and springtime had brought more parties. But the numbers were growing too quickly to easily explain, prompting a distressing question.

“Is this a different strain?” one hospital official asked in a group email in early November, raising the possibility that the virus had developed a dangerous mutation.

A small but rigorous clinical trial in Argentina has found that blood plasma from recovered Covid-19 patients can keep older adults from getting seriously sick with the coronavirus — if they get the therapy within days of the onset of the illness.