Tomas Pueyo published an article: https://medium.com/@tomaspueyo/coronavirus-act-today-or-peop...
He wrote a model that roughly estimates infection rates in a population based on either deaths or confirmed cases: https://docs.google.com/spreadsheets/u/1/d/17YyCmjb2Z2QwMiRR...
The model is oriented around companies, but could be applied to any gathering of people. There may be more sophisticated or accurate models out there, but I found his relatively quick to understand and apply.