Temporal Changes in Pediatric Influenza Test Positivity Across COVID-19 Public Health Policy Phases: A Five-Year Retrospective Cohort Study

Background The COVID-19 pandemic coincided with substantial changes in respiratory virus circulation, healthcare utilization, and diagnostic testing. Although population-level influenza activity declined markedly in many settings during periods of stringent public health restrictions, patterns among children undergoing clinical testing remain less clearly understood. This study compared pediatric influenza test positivity across pre-pandemic, restriction-dominant pandemic, and post-restriction periods. Methods This retrospective cohort study included 18,035 children who underwent influenza A/B antigen testing at the Department of Pediatrics, Hisar Intercontinental Hospital, Istanbul, Türkiye, between January 2018 and May 2023. The study period was divided into the pre-pandemic (January 2018-February 2020), restriction-dominant pandemic (March 2020-June 2021), and post-restriction (July 2021-May 2023) periods. Influenza A and B antigens were detected in nasopharyngeal swab specimens using the GenBody Influenza A/B Ag rapid immunochromatographic assay. Tests performed for the same patient within 14 days were considered part of the same illness episode, and only the first eligible test was included. Influenza test positivity was compared across periods and according to age, sex, influenza type, and seasonality. Multivariable logistic regression was used to evaluate factors independently associated with influenza positivity. Results Of the 18,035 children, 6,691 were tested during the pre-pandemic period, 3,375 during the restriction-dominant period, and 7,969 during the post-restriction period. Influenza was detected in 1,699 (25.4%), 969 (28.7%), and 1,937 (24.3%) children, respectively (P<0.001). Although the proportion of positive tests was highest during the restriction-dominant period, both the testing volume and absolute number of detected cases were lower than before the pandemic. Influenza A positivity increased from 17.1% during the restriction-dominant period to 20.0% after restrictions were relaxed, whereas influenza B positivity declined from 11.6% to 4.4%. Seasonal positivity also varied substantially across the study periods. After adjustment for demographic characteristics, the epidemiological period remained independently associated with influenza test positivity. Conclusions Temporal changes in pediatric influenza test positivity coincided with changes in COVID-19 public health measures. The higher positivity among tested children during the restriction-dominant period occurred in the context of substantially reduced testing and should not be interpreted as increased population-level influenza circulation. Changes in healthcare utilization, testing behavior, and the composition of the tested population should be considered when interpreting these findings.