To see the damage from covid-19 to aviation, look up. Where once a criss-cross of vapour trails told of holidaymakers heading for the sun or executives keeping businesses on track, the wide yonder is now a brilliant blue. This year nearly 5bn passengers might have been expected to take to the skies, but the actual number is likely to be only half as big. A fragile recovery is susceptible to new waves of infection. Britain’s imposition this week of a quarantine on passengers returning from Spain is the latest setback. Traffic may not return to 2019 levels until 2024.