In May 2022, a national control program targeting porcine reproductive and respiratory syndrome virus was launched in Denmark, and the first regions are approaching a PRRS-free status. Hence, the question now arises as to how the surveillance should be performed to identify new incursions of the PRRS virus in PRRS-free regions as early as possible. The aim of the present study was to quantify the early detection sensitivity of the current and alternative PRRS surveillance systems in Denmark at a regional level. The current PRRS surveillance system is composed of a notifiable surveillance component and an active serological surveillance component, consisting of monthly sampling (breeding and multiplier farms) or yearly sampling (production farms). The results from the present study estimated that the current surveillance system would have a 21.5 % [16.0 %
32.4 %]