Let us look first at St Peter's Basilica.
"The risk factors include the dimensions of the structure, its location and its location relative to other buildings around it," explains Matthew Waldrum from Omega Red Group, who assesses the risk of lightning hitting buildings.
"The construction material of the building is obviously also very important.
"It's an extremely large structure. It's not exactly isolated, but it certainly towers over its surroundings, which means that lightning is more likely to hit it than any of the surrounding buildings."
The fact that no damage occurred to the basilica suggests that it is quite well protected against lightning.
But Waldrum and his colleagues carried out a theoretical calculation of the risk of loss of life due to lightning in a similar building frequented by large numbers of people, in a similar setting - assuming the lack of any lightning protection.