In this Aug. 24, 2020 file photo, a student, wearing a face mask and shield, returns to the Melpark Primary School in Johannesburg after several months of lockdown. Ordinarily, South Africa sees widespread influenza during the winter months, but this year almost none have been found — something unprecedented. School closures, limited public gatherings and calls to wear masks and wash hands have “knocked down the flu,” said Dr. Cheryl Cohen, head of the institute’s respiratory program. (AP Photo/Denis Farrell, File)

In this Aug. 24, 2020 file photo, a student, wearing a face mask and shield, returns to the Melpark Primary School in Johannesburg after several months of lockdown. Ordinarily, South Africa sees widespread influenza during the winter months, but this year almost none have been found — something unprecedented. School closures, limited public gatherings and calls to wear masks and wash hands have “knocked down the flu,” said Dr. Cheryl Cohen, head of the institute’s respiratory program. (AP Photo/Denis Farrell, File)