Mass Gatherings As Amplifiers of Meningococcal Disease: A Modeling Study of Hajj and Umrah.

The Kingdom of Saudi Arabia (KSA) hosts millions of Hajj and Umrah pilgrims annually, and it aims to welcome 30 million external (international) Umrah pilgrims annually by 2030 (Saudi Vision 2030). These mass gatherings pose significant risks for infectious disease transmission. We extended a previous meta-population model of Neisseria meningitidis transmission during Hajj to include Umrah pilgrimage and assessed the impact of varying vaccination coverage rates (VCRs) among pilgrims on meningococcal meningitis (MM) cases. The model population was divided into five clusters: residents of the pilgrimage sites (Makkah/Madinah); the broader KSA population; and non-KSA populations from high-, medium-, and low-transmission settings. The model incorporated age, pilgrim movement patterns, and VCRs. The model was calibrated with data from 1995 to 2011 and validated with data from 2012 to 2024. MM cases under various vaccination and pilgrim scenarios were simulated for the 2025–2034 period. Umrah accounted for approximately 90