Evaluation of an influenza-like illness sentinel surveillance system in South Korea, 2017-2023

Background: Guided by the data from the surveillance system, public health efforts have contributed to reducing the burden of influenza in many countries. During the COVID-19 pandemic, many surveillance resources were directed at tracking the severe acute respiratory syndrome-Coronavirus 2. However, most countries have not reported surveillance evaluations during the COVID-19 pandemic. Methods: Using the U.S. CDC surveillance evaluation method, we evaluated the influenza-like illness (ILI) sentinel surveillance performance in South Korea between January 2017 and September 2023. For the timeliness, we measured the mean time lag between the reports from the sentinel sites to the Korea Disease Control and Prevention Agency (KDCA) and surveillance result dissemination from KDCA. For the completeness, we measured the submission rate of complete reports per overall number of reports from each sentinel site to the KDCA. For the sensitivity, we calculated the correlation coefficient between the monthly number of ILI reports and the patients with ILI from the Korea national reimbursement data by either Pearson's or Spearman's test. For the representativeness, we compared the age-specific distribution of ILI between the surveillance data and the national reimbursement data using a chi-squared test. Results: We found that the surveillance performance of timeliness (less than 2 weeks) and completeness (97 %-98 %) was stable during the study period. However, we found a reduced surveillance sensitivity (correlation coefficient: 0.73 in 2020, and 0.84 in 2021) compared to that of 2017-2019 (0.96-0.99), and it recovered in 2022-2023 (0.93-0.97). We found no statistical difference across the proportion of age groups between the surveillance and reimbursement data during the study period (all P-values > 0.05). Conclusions: Ongoing surveillance performance monitoring is necessary to maintain efficient policy decision-making for the control of the influenza epidemic. Additional research is needed to assess the overall influenza surveillance system including laboratory and hospital-based surveillance in the country.