Pandemic-Related Reductions in Respiratory Infections and New-Onset Nephrotic Syndrome: A Multicenter Cohort Study in Japan.

Rationale & Objective: Infectious diseases have been associated with pediatric idiopathic nephrotic syndrome (NS). The effect of pandemic-related changes in infection exposure on NS epidemiology remains unclear. Study Design: Retrospective cohort study. Setting & Participants: Data from children aged 6 months to 15 years from 11 hospitals in Japan, extracted from the Pediatric Medical Information Collection System. Two periods were compared: prepandemic (April 2019-March 2020) and first pandemic year (April 2020-March 2021). Analytical Approach: Event counts (new-onset NS, relapse, respiratory tract infections, acetaminophen prescriptions, coronavirus disease 2019 diagnoses) were modeled using a negative binomial generalized linear mixed model with hospital-specific random intercepts. Sensitivity analyses used zero-inflated Poisson, Poisson mixed models, and quasi-Poisson. Results: There were 218 new-onset NS cases (127 prepandemic, 91 during the pandemic). The pandemic period was associated with a decrease in incidence, and estimates were similar across distributional sensitivity analyses, whereas extended time-window analyses attenuated statistical significance. The relapse rate among 366 patients in remission was 0.094 per person-year before the pandemic and 0.056 per person-year during the pandemic; the period effect was not statistically significant. Respiratory tract infections decreased by approximately half, and acetaminophen prescriptions declined. The number of coronavirus disease 2019 diagnoses increased sharply. Limitations: Observational design limits causal inference. Findings may not generalize beyond Japanese populations. Conclusions: Reduced respiratory infection exposure during the pandemic may be associated with lower new-onset NS incidence. Preventive measures against respiratory infections may help reduce NS occurrence.