In some parts of the US, South Dakota to be specific, if 10 people get together, there is a 50% chance one of them has Covid. If people are selected randomly from the whole of the US, that number increases to 47 for the same 50% chance that one of them has it. We do know that indoors is high risk, especially if for a long time, close together, and if alcohol is involved - loud voices/shouting/singing.

It would be interesting if there was a risk calculator for the UK - put in the number of people, where they are from, and see what the risk of a xmas get-together would be.