Paternal Health and Offspring Sex Ratio During the COVID-19 Pandemic in South Korea: Does Y-Sperm Vulnerability Explain the Shift?

Impact of the COVID-19 Pandemic on the Sex Ratio at Birth in South Korea: A Nationwide Population-Based Study 1. Background and Purpose The human sex ratio at birth (SRB) is generally stable but can fluctuate under severe societal or environmental stressors, a phenomenon often explained by the Trivers–Willard hypothesis. The COVID-19 pandemic served as a profound global stressor, yet evidence regarding its impact on the SRB remains heterogeneous across different countries. This project aims to evaluate temporal changes in the SRB in South Korea during the COVID-19 pandemic using national registry data. Specifically, we investigate whether acute societal stress alters the sex composition of newborns, with a particular focus on Daegu, the epicenter of South Korea’s first major outbreak. We test the hypothesis that stress-induced perturbations act through conception-linked mechanisms (e.g., a "paternal filter" affecting Y-bearing sperm) rather than purely gestational loss, by examining time lags of approximately nine months after infection peaks. 2. Methodology • Study Design: Retrospective population-based study. • Data Source: Aggregate live birth data from Statistics Korea (KOSTAT) covering January 2015 to December 2022. • Analysis: We employed a three-stage statistical approach: 1. Chi-squared tests to compare pre-pandemic (2015–2019) and pandemic (2020–2022) periods. 2. Interrupted Time Series (ITS) analysis to assess longitudinal shifts in level and slope. 3. Baseline-derived 95% prediction intervals to identify month-specific anomalies and regional variations. 3. Key Findings and Expected Outcomes This project provides empirical evidence of a statistically significant decline in the nationwide SRB in South Korea during the pandemic. • Primary Finding: In Daegu, a distinct trough in the proportion of male births was observed in November 2020. This occurred exactly nine months after the region's peak infection wave (February–March 2020), supporting a conception-timed mechanism driven by acute psychosocial and environmental stress. • Secondary Finding: A further suppression of the SRB was identified in early 2022, coinciding with the "4th Wave," vaccination rollout, and prolonged pandemic fatigue, suggesting a complex interplay of physiological (e.g., fever) and behavioral (e.g., preconception delay) factors. • Implication: The study establishes the SRB as a sensitive biological indicator of population distress and underscores the necessity of integrating reproductive health protection into public health responses during large-scale crises. 4. Data Availability This repository contains the cleaned dataset (korea_births_long_full.csv) and the R source code used to generate the statistical results and visualizations (Figures 1–4) presented in the associated manuscript.