The Denominator problem: estimating the size of local populations of men-who-have-sex-with-men and rates of HIV and other STIs in Switzerland.

Combining general population data, MSM online surveys and notification data allows the calculation of realistic estimates of local MSM populations and thus proportions of MSM with diagnosed HIV and other STIs, with implications for prevention planning, commissioning of health services and counselling MSM on HIV/STI risk. Our methodology for Switzerland is transferable to other countries with similar data sources.