Investigating novel approaches to tick-borne encephalitis surveillance in Sweden, 2010-2017

Tick-borne encephalitis (TBE) is a vaccine-preventable, high-priority disease in Sweden, with increasing in-cidence. However, surveillance is limited to case reports. We investigated relationships between reported TBE incidence and syndromic surveillance data to determine if these novel data sources could provide earlier in-dications of disease activity. We retrospectively compared national, weekly (2010-2017) reported TBE in-cidence to the percentage of TBE-related a) searches on the main Swedish healthcare information website and b) calls to its telehealth service using Spearman's rho to determine the most strongly correlated lags. We conducted a sub-analysis (2012-2017) of TBE-related Google Trends queries and compared the number of TBE-related media stories to each novel surveillance dataset. Healthcare website searches for tbe and vaccine combined, tbe, tick, and tick bite led case data by 12, 8, 7, and 6 weeks, respectively (rho = 0.87-0.89); telehealth calls led by 4 weeks (rho = 0.92; all p < 0.001). Correlations and lags for Google Trends and healthcare website searches were fairly similar to each other. In comparison, correlation between the different syndromic sur-veillance datasets and the number of media stories was lower (rho = 0.25-0.56). We observed volume dis-crepancies between TBE incidence and the novel surveillance datasets during some years, particularly for web searches. Syndromic surveillance data were strongly correlated with and preceded case data by 4-12 weeks. Syndromic data may provide advanced awareness and earlier indications of TBE activity, which can improve timing and specificity of public health communications. The use of these data as supplements to notifiable disease data for national planning and preparedness in real-time should be investigated.