Hemisphere Flip in Season-of-Birth Disease Associations: A Multi-State Brazilian Hospitalization Study

This study tests whether birth-month disease associations found in Northern Hemisphere populations (NYC, Boland & Tatonetti 2015) flip by approximately 6 months in the Southern Hemisphere, as predicted by a seasonal/vitamin D mechanism. Using publicly available Brazilian hospitalization data (DATASUS SIH-SUS), we conducted a phenome-wide Season-Wide Association Study (SeaWAS) across 30 ICD-10 disease categories in 5 Brazilian states (SP, RJ, MG, RS, BA), covering 36.9 million hospitalizations from 2015-2022. We identify 9 diseases with statistically significant hemisphere flips (peak birth month shift of 5-7 months, p < 0.05 after Bonferroni correction), consistent with a vitamin D seasonal hypothesis. The flip replicates across all 5 states for the strongest associations. Diseases showing no flip suggest non-seasonal developmental mechanisms. Data and analysis code are fully reproducible from public DATASUS FTP.