Estimated Number of Symptomatic Lyme Borreliosis Cases in Adults in Finland in 2021 Using Seroprevalence Data to Adjust the Number of Surveillance-Reported Cases: A General Framework for Accounting for Underascertainment by Public Health Surveillance.

Finland conducts public health surveillance for Lyme borreliosis (LB) based on clinically diagnosed and laboratory-confirmed cases. We used data from seroprevalence studies to determine the extent to which LB cases were underascertained by public health surveillance. The numbers of incident symptomatic LB cases in 2011 in six regions in Finland were estimated using (1) data from sensu lato seroprevalence studies, (2) estimates of the proportion of LB infections that are asymptomatic, and (3) estimates of the duration of LB antibody detection. The numbers of estimated incident symptomatic LB cases were compared with the numbers of surveillance-reported LB cases to estimate regional underascertainment multipliers. Underascertainment multipliers were applied to the numbers of surveillance-reported LB cases in each region in 2021 and summed to estimate the number of symptomatic LB cases in Finland among adults in 2021. A sensitivity analysis evaluated the impact of different durations of antibody detection. Using an asymptomatic proportion of 50% and a 10-year duration of antibody detection, the estimated regional underascertainment multipliers in Finland ranged from 1.0 to 12.2. Applying the regional underascertainment multipliers to surveillance-reported LB cases in each region and summing nationally, there were 19,653 symptomatic LB cases in Finland among adults in 2021 (526/100,000 per year). With 7,346 surveillance-reported LB cases in Finland among adults in 2021, the estimated number of symptomatic LB cases indicate that there were 2.7 symptomatic LB cases for every surveillance-reported LB case among adults. With a 5- or 20-year duration of antibody detection, there were an estimated 36,824 or 11,609 symptomatic LB cases among adults in 2021, respectively. Finland has robust public health surveillance for LB, but cases are underascertained. This framework for estimating LB underascertainment can be used in other countries that conduct LB surveillance and have conducted representative LB seroprevalence studies.