Professor Philip Nolan:

It is very important to state that the evidential base for this is strong. If we congregate indoors, the risk of transmission is high among adults. If we congregate closely outdoors and do not maintain physical distance of 2 m or more, the risk is moderate to low. The big issue here is not about spectatorship. This is about congregating in numbers greater than 15 outdoors, particularly where people from different households are freely mixing. The very purpose of the recommendation is to allow important activities, such as participation in sports or schooling, to continue. I say that because every contact between two adults that carries the risk of viral transmission carries that risk. The whole purpose of the recommendation is to prevent indoor and outdoor congregation that fosters mixing between households that would spread the virus between households. It is not targeted at any activity but at open mixing between people from different households. The Chairman is correct that the risk is lower outdoors, but it is not negligible outdoors, particularly where people understandably let their guard down.