Concerns about school infrastructure are adding momentum to plans in some districts in the US, even in colder climates, to take classes outdoors for the sake of student and teacher health. Nationwide, an estimated 41 per cent of school districts need to update or replace their heating, ventilation and cooling systems in at least half their schools, a federal report issued in June stated.

Ineffective systems a threat

There is no proof that the disease can spread through ventilation systems from one classroom to the next, said Dr Edward Nardell, a Harvard Medical School professor who specialises in airborne diseases. The danger, he said, is from ineffective systems that don't remove floating viruses and let them linger in classrooms after they are expelled in an infected person's breath or sneeze.