A Clear Case of Outdoor Transmission

While the vast majority of superspreading events took place outdoors, there are also dozens of superspreading events that took place in settings with both indoor and outdoor elements: For example, there were large demonstrations in Madrid and a soccer match in Milan between Valencia and Atalanta Bergamo that are suspected superspreading events.

But while these specific events took place outdoors they also involved a lot of indoor activity, such as transportation to and from the event, and socializing in bars and restaurants before and after. In such cases it is difficult to prove whether transmission occurred in the indoor or in the outdoor part.

Clear examples of outdoor transmission have been hard to come by. There was a runner in Italy who infected his running partner, but there is too little information known about that case to say for sure whether transmission occurred during the run or during indoor activities before or after.

There is, however, at least one unambiguous example of outdoor transmission. Researchers studied 7,324 Covid cases in China and found one case that involved only outdoor activity: A long conversation between two friends —one of whom had just gotten back from Wuhan.

So they do exist.

But the fact that clear examples of outdoor transmission are so rare while clear examples of indoor transmission so abundant does suggest —but not prove— that in cases with both indoor and outdoor elements, it seems more likely that transmission occurred in the indoor part.