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% of pilgrimage days in 2023–2024, with external pilgrims accounting for > 90% of these days. In a scenario where external pilgrim numbers increased in line with Saudi Vision 2030, the probability of an annual number of MM cases exceeding 100 among pilgrims during the 2025–2034 period was estimated at 53% with a 10% VCR among internal (domestic) and external Umrah pilgrims versus < 0.02% if both VCRs were 90%. High VCRs (90%) for external and internal pilgrims reduced the number of expected MM cases across the KSA by > 80% compared with no vaccination. Our analysis highlights the amplifying effect of mass gatherings on disease transmission dynamics and confirms vaccination as a highly effective intervention. Strengthened enforcement of meningococcal ACWY vaccination requirements for all Umrah pilgrims would significantly reduce MM outbreak risk and benefit the entire KSA population, as well as the pilgrims and people in their countries of origin. Every year, millions of people travel to the Kingdom of Saudi Arabia (KSA) to perform the Hajj and Umrah pilgrimages. Large gatherings like these can increase the spread of infectious diseases, including meningococcal meningitis, a serious infection that affects the membranes surrounding the brain and spinal cord. In this study, we expanded an existing disease transmission model originally developed to examine the risk of meningococcal meningitis associated with Hajj pilgrimage. We included new data that described the number of domestic and international Umrah pilgrims, their travel patterns, how long they stayed in the KSA, and the proportion of people who were vaccinated against meningococcal disease. We used this updated model to estimate the number meningococcal meningitis cases that could occur among pilgrims and within the KSA with different vaccination rate scenarios. Our results show that the percentage of meningococcal meningitis cases prevented among pilgrims is mainly influenced by the proportion of international pilgrims who are vaccinated. Conversely, the percentage of meningococcal meningitis cases prevented in the KSA is mainly influenced by the proportion of domestic pilgrims vaccinated. Our findings highlight the need to improve Umrah pilgrims’ vaccination rates to protect them against meningococcal meningitis, as well as protecting people in the local communities at the holy sites, across the KSA, and in the home countries of returning pilgrims.