Background:
What have we learned about surveillance during the pandemic? Surveillance systems drive policymakers’ decisions, but it is not easy to transform data into useful information. Multiple surveillance methods are needed, and clear definitions of what is being monitored are fundamental.

Objectives and Key Questions:
The objective of this workshop is to strengthen the culture of surveillance and discuss the strengths and weaknesses of various COVID-19 surveillance strategies. Key questions will be: 1) which information is needed for decision-making? 2) how can we monitor the pandemic and its impact? 3) how should we adapt the system at different stages of the pandemic?

Overview:
We will have a short introduction and 4 case studies (each 10 min) followed by an active discussion. Introduction (Chiolero and Stranges) Multiple challenges hampered COVID-19 surveillance (lack of common standards, different diagnostic strategies, infodemia). The pandemic has highlighted the advantages and disadvantages of different surveillance strategies and the importance of having a multilayer and highly adaptive health information system. Case study 1: Seroprevalence studies for the surveillance of COVID-19: Corona Immunitas (Tancredi) Population-based seroepidemiological studies are needed to understand SARS-CoV-2 spread. We will present the methods and results of Corona Immunitas, a Swiss national research project consisting of repeated population-based serological studies conducted with a common methodology. The project included a Digital Follow-Up eCohort to monitor health status changes, adherence to preventive measures, and the social impact of the pandemic. Case study 2: Challenges in the definition of long COVID or Post-COVID-19 condition (Makovski) We will present an operationalization of the WHO definition of the post-COVID-19 condition on a sample of the general adult population in France. We will describe the prevalence of each symptom at different time points after the infection and their correlation, and compare characteristics of post-COVID-19 condition subjects (as selected by WHO definition) with those not selected but who perceived experiencing Long COVID. Case study 3: Electronic health records to assess vaccine effectiveness (Bejko) Linked routinely collected electronic health records at the national level can help evaluate vaccine effectiveness. Using data from laboratory confirmed SARS-CoV-2 PCR tests linked with vaccination data and SARS-CoV-2 hospitalisations and deaths, we investigated severe outcomes (hospitalisation, death) in relation to variants, vaccination status, and prior infection and evaluated the effectiveness of a fourth bivalent Omicron-containing booster dose and natural immunity due to prior infection against laboratory confirmed SARS-CoV-2 infection. Case study 4: A geospatial surveillance portal to monitor COVID-19 transmission (Gilliland and Stranges) Based on a geospatial portal for real-time surveillance of COVID-19 transmission and detection of disease clusters, we demonstrated the use of emerging hot spot analysis techniques to map and analyze spatiotemporal changes in COVID-19 transmission in Middlesex County, Ontario, between July 2020 and January 2021. The project demonstrated that geospatial surveillance portals are valuable tools for helping public health decision makers to identify and respond to disease outbreaks. Discussion (Chiolero and Stranges)