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.