Piloting of a surveillance system for acute respiratory diseases: COVID-19 monitoring using Sick Leave Certificates.

With the end of the COVID-19 pandemic and the decreasing significance of official reporting figures, the Lower Saxony State Health Office developed and tested a new indicator: the "7-day sick leave incidence". Unlike previous surveillance indicators, it is intended for syndromic surveillance of COVID-19. This article explains the methodological development as well as its benefits, possible applications, and limitations.The indicator is based on the weekly number of sick leaves due to COVID-19 per 100,000 health insurance members entitled to sickness benefits (KGbM) of the AOK Lower Saxony (AOKN). The development of the indicator involved differentiating between initial and follow-up sick leaves, investigating fluctuations in the number of KGbM, analysing the doctors' assignments of ICD Codes U07.1! and U07.2!, and ensuring the timely availability of sick leave data.Initial and follow-up sick leaves were distinguished using a temporal algorithm. In 2022 and 2023, on average, 83.0% (s=5.4%) and 88.9% (s=2.3%) of all initial COVID-19-related sick notes were submitted on time by the end of the respective calendar week. Four out of 5 initial sick notes contained the doctors' ICD code U07.1! (lab-confirmed COVID-19). The number of KGbM proved to be stable (M=1.218.202, s=11.003). When comparing the new "7-day sick leave incidence" with the officially used "7-day incidence rates" during pandemic, trends were highly similar in 2022 (r=0.89), but diverged significantly in 2023 (r=0.26) due to declining diagnostic activities for the "7-day incidence rates".The new 7-day-sick-leave incidence is a good representation of the post-pandemic COVID-19 infection dynamics. The indicator uses routine data and is easy to establish. Limitations relate to possible changes in diagnostic procedures, doctors' coding behaviors and changing demands for sick leave.