The Arbaeen pilgrimage, one of the largest religious gatherings, draws millions of pilgrims from around the world each year, particularly at the Mehran border crossing, the most important transit point to Iraq, presenting a unique display of unity, faith, and solidarity (1, 2). This massive event, characterized by extreme population density, close physical contact, diverse geographical origins of participants, and challenging environmental conditions, significantly increases the risk of infectious disease outbreaks, including respiratory infections, diarrheal diseases, and food- and waterborne illnesses (3–5). Studies have shown that factors such as age, gender, health conditions, and limited access to medical facilities play significant roles in the distribution and severity of various diseases (6–8). Without proper supervision, such gatherings might become potential hotspots for the rapid transmission of infections (9, 10).

Managing infectious diseases during such events requires thorough preparation, interagency collaboration, and the implementation of effective preventive measures (11, 12). The deployment of temporary health centers, monitoring of water and food quality, distribution of hygiene kits, and pilgrim education are some of the steps that have helped limit the likelihood of illness outbreaks (13, 14). However, issues such as poor health facilities, insufficient waste management, and excessive overcrowding at the Mehran border continue to pose substantial barriers (15, 16). The analysis of pilgrims' demographic data, including age, gender, and geographical region of origin, enables the identification of high-risk groups and the formulation of tailored interventions (17, 18).

Between 2022 and 2024, public health services near the Mehran border implemented measures to enhance sanitary infrastructure, establish monitoring protocols, and deploy mobile medical teams (19, 20). The Arbaeen Health project, which provided safe drinking water, conducted illness screenings, and offered vaccination services, significantly contributed to reducing the incidence rate of communicable diseases (21, 22). Nonetheless, qualitative studies indicate that inter-agency cooperation, resource allocation, and health worker training require strengthening (11). Recent evaluations suggest that using early detection algorithms and improved surveillance systems could yield more sustainable benefits in disease control (1, 2).

The Arbaeen pilgrimage presents both a public health concern and a unique opportunity for innovative global health management. The efficacy of preventive efforts during this event gives an encouraging template for other large gatherings, such as the Hajj or significant sporting and cultural events (3, 8). In an era where rising challenges, such as antimicrobial resistance and novel diseases, transcend boundaries, Arbaeen teaches us that through collaboration, research, and a commitment to collective health, we can take deliberate steps toward a better world, even under the most complex conditions (20, 22).

This study sought to address infectious disease management by examining demographic data from pilgrims, evaluating the performance of public health systems, and estimating the efficacy of preventive initiatives at the Mehran border between 2022 and 2024. The study's findings can help improve health management techniques for large religious gatherings, as well as provide actionable insights for global communicable disease prevention and control.

A thorough census-based strategy was used, with all patients presenting with syndromic symptoms, such as respiratory, gastrointestinal, febrile, and other pertinent clinical manifestations, being comprehensively investigated within the given timeframe. To guarantee proper disease confirmation, symptomatic patients provided laboratory samples (including PCR testing for COVID-19 and influenza, fast tests for cholera (El Tor), and targeted bacterial cultures. All favorable results were documented in the integrated health surveillance system.

Three key criteria decided eligibility for the study:

Data were compiled from two key sources: electronic registration systems, which included standardized syndromic surveillance forms (documenting clinical symptoms, syndrome classification, and diagnostic test findings), and daily border crossing records of pilgrims entering and leaving Mehran.

Field reports from health teams, which work three shifts each day (morning, afternoon, and night), include operational data such as water source chlorination levels, food quality monitoring records, and hygiene kit distribution statistics. All data were cleaned, consolidated, and validated before being organized into a single database for statistical analysis. Study Variables:

Demographic Variables: Total pilgrim volume, entry/exit patterns, participant nationalities, and daily population density at the border.

Clinical Variables: The frequency of syndromic cases (respiratory, gastrointestinal, and febrile), the number of laboratory samples obtained, and the positivity rates (confirmed cases relative to total samples) were determined using diagnostic procedures such as PCR and fast testing.

Performance Indicators: The ratio of symptomatic patients to total pilgrims, laboratory confirmation rate, and annual trends in these measures are used to assess the effectiveness of the intervention and detect epidemiological patterns across the various phases of the ceremony.

This descriptive-analytical study utilized secondary data extracted from official registries and field reports. According to the Evidence-Based Medicine (EBM) classification system, the generated evidence corresponds to Level III (non-experimental analytical studies, including correlational or case-control designs).

The integrated methodology, which combines demographic, clinical, and operational data, enables a precise evaluation of the effectiveness of public health interventions in controlling infectious diseases during mass gatherings. This analytical framework not only identifies actual epidemiological patterns but also establishes a standardized model for health monitoring and risk management in future similar events.

1. Demographic Data and Volume of Health Services

Over the three-year study period (2022–2024), 1.9 million pilgrims passed through the Mehran International Border Crossing, with 7.4 million entering and 4.4 million leaving. The most significant traffic concentration was observed during the key 7-day period surrounding Arbaeen (three days before to three days after the main celebration), when 85 percent of all healthcare visits occurred. The largest operating load on the health system occurred on the day of Arbaeen in 2023, when 173217 people got healthcare services, demonstrating a major strain on the health infrastructure during high-density pilgrimages. The daily average of healthcare supplied over the Arbaeen period is estimated at 76150. (Table 1, Table 2).

In this multi-year, real-world surveillance study at a major international border crossing during the massive Arbaeen gathering, we observed significant temporal shifts in the syndromic disease burden, the intensity of public health interventions, and health system performance between 2022 and 2024. Our research reveals a complex interplay between passenger volume, preventive capacity, and the prevalence of infectious diseases at one of the world's largest recurring religious mass gatherings.

The current study is one of the most comprehensive field evaluations of the efficacy of an Integrated Syndromic Surveillance System at the Mehran border crossing, which serves as the primary gateway to one of the region's largest religious gatherings. Our results, based on monitoring over 9 million pilgrims over three years (2022–2024), demonstrate that the strategic integration of environmental health interventions, public education, and human resource strengthening has significantly shifted the epidemiological pattern of communicable diseases in favor of public health (Figs. 6 and 7). A 91 percent reduction in confirmed El Tor cases and a 60 percent reduction in respiratory syndromes in 2024, compared to the previous year, not only marks a success for the Iranian health system but also elevates the Mass Gatherings Health management model from a reactive to a preventive and proactive approach (1, 2). These findings align with similar studies in religious gatherings, such as the Hajj (6, 7), and Kombe Mela (2024), as reported by the World Health Organization (21), which emphasise the crucial role of syndromic screening and the distribution of hygiene kits (3).

One of our study's most notable findings was the sinusoidal pattern of respiratory cases, which showed an increase in 2023 followed by a sharp decrease in 2024. This initial surge can be attributed to 'post-pandemic fatigue' and decreased adherence to health protocols following the lifting of COVID-19 restrictions. These data are consistent with findings during the Hajj and the Arbaeen pilgrimage, which reported a severe rebound of respiratory viruses due to decreased protective behaviors and high population density (1, 2). However, unlike many other religious meetings where disease rates remained high after the pandemic, vigorous action in 2024 (including mask distribution and targeted immunization) was successful in interrupting the chain of transmission. The 60 percent reduction in respiratory infections in our study suggests that, even in open, dusty situations with high population dynamics, 'personal protection' remains the most significant protective barrier. This finding is totally consistent with prior research that emphasizes the use of masks in decreasing respiratory illnesses among Hajj travelers (1–3). The most noteworthy accomplishment of this study was the prevention of a potential cholera outbreak in a setting where neighboring nations were suffering intermittent outbreaks. Unlike the 'Kumbh Mela' in India, where bathing in the river is a significant risk factor for waterborne diseases (27–29), the Arbaeen model at the Mehran border, which focuses on environmental health engineering interventions such as safe packaged water and continuous chlorination, has reduced environmental transmission risk. This achievement verifies the premise of Memish et al., who claim that engineering interventions are preferable than therapeutic ones in large groups (4, 5). Our correlation analysis demonstrated that face-to-face instruction and the distribution of hygiene items were not only supplementary measures, but rather a fundamental intervention in lowering gastrointestinal disorders. In terms of resource management, our analysis revealed that a 72% increase in human resources in 2023 led to a 54% rise in case detection. This conclusion is methodologically noteworthy since it implies that portion of the fluctuation in disease statistics at mass meetings is attributable to "increased surveillance sensitivity," rather than an actual increase in disease incidence (6). This distinction is crucial in understanding epidemiological data because it demonstrates that our strategy of deploying mobility teams has resulted in the identification of mild cases, thereby preventing them from becoming sources of transmission. Also, comparing the COVID-19 test positivity rate (12%) at the Mehran border to the findings of Suraifi et al. (2024), who reported a 48.8% rate of influenza-like illness syndromes on Iraqi pedestrian routes, suggests the usefulness of preventive screening at the zero-point border (6–12).

Furthermore, the concentration of 85 percent of services within 7 days around Arbaeen emphasizes the concept of "surge capacity." Our findings indicate that the health system at zero-point borders requires a flexible framework capable of rapidly increasing its capacity within 24 hours, which is compatible with the WHO-proposed models for disaster preparedness (13, 14). Despite its virtues, this study has certain limitations. The cross-sectional structure of the data prevented researchers from investigating the long-term results of pilgrims when they returned to their starting point. Furthermore, due to excessive overcrowding, it was not possible to conduct PCR tests on all suspected respiratory patients; hence, diagnoses relied on syndromic presentation. However, the three-year consistency of trends, as well as the 9-million sample size, confirms the epidemiological validity of the findings.

The processes underlying the observed changes are likely multifaceted. The high positive association between passenger volume and syndrome occurrence (e.g., r = 0.78 with acute watery diarrhea) indicates that density is a key amplifier of transmission risk (15, 16). In contrast, the negative correlation between preventive measures (mask distribution/education) and illness burden suggests a protective impact; however, ecological data do not allow for causality at the individual level. ⁹ The 54 percent rise in case identification following a 72 percent growth of human resources in 2022 highlights the fundamental relevance of staffing levels in surveillance sensitivity, a notion that remains even under severe conditions (17–21).

These findings have important implications for public health and policy. First, the 85 percent concentration of health service visits within a 7-day window surrounding Arbaeen needs a surge capacity model for border health systems, with flexible, pre-positioned human and material resources that can quickly scale (6, 9, 18, 20). Second, the victory against cholera shows that controlling fecal-oral infections in such circumstances is possible with concerted WASH measures. Third, the ongoing threat of respiratory infections necessitates the incorporation of routine genetic surveillance to track variations and inform vaccination or chemoprevention measures (2–4, 6, 11, 15, 17, 18, 22–26). Finally, despite favorable results, the rapid reduction in overt preventative activities in 2023 serves as a warning not to perceive reduced effort as enhanced efficiency in the absence of robust, contemporaneous outcome monitoring. Health authorities must ensure that savings do not lead to the next disaster.

The success of this model can be analyzed using a triple mechanism framework that includes epidemiological (we hypothesize that the border surveillance system acted as a biological barrier), systems perspective (we hypothesize that the health system effectively operationalized the surge capacity mechanism across the four domains of Staff, Stuff, Structure, and Systems (4 Ss), and behavioral level (face-to-face education played a vital role in modifying pilgrims' (20, 27–29). These findings have significant strategic implications for health policymakers, demonstrating that health surveillance at border crossings should be considered an essential component of global health security. On the other hand, the current study findings must be interpreted in light of several limitations. The use of aggregated data precludes the analysis of individual-level variables, and the ecological study design does not permit definitive causal inference between a specific intervention and disease reduction. The ephemeral nature of pilgrims at the zero-point frontier may also result in under-ascertainment. Despite these limitations, the large sample size and three-year longitudinal follow-up provide significant statistical power. Finally, our findings give empirical evidence that controlling health in large crowds using an integrated model of syndromic surveillance and increased surge response capacity is not only feasible but also necessary for defending public health in the twenty-first century.

Notable qualities include a significant population-based sample collected over three years, providing a unique longitudinal perspective on a massive, recurring gathering. The integration of operational, clinical, and demographic data from a border health system under real-world situations gives high external validity for comparable scenarios (24, 30). Furthermore, using laboratory confirmation for major infections (such as COVID-19, El Tor, influenza, measles, and dengue) improves the accuracy of trend studies for these diseases.

Finally, the strategy established at the Mehran border can serve as a model for cross-border health diplomacy around the world. Success in disease control shows that managing biological hazards during international gatherings necessitates a transition from passive to active surveillance and data sharing between source and destination countries. Arbaeen is more than just a religious event; it is also a living epidemiological laboratory, demonstrating that global health security can be achieved through the intelligent integration of meticulous surveillance, environmental interventions, and resource mobilization, even under challenging conditions.

This study found that implementing an integrated model of active surveillance, health education, and environmental interventions was effective in controlling communicable diseases at one of the world's largest public gatherings. During the study period, we found that this technique reduced cholera (El Tor) cases by 91 percent and respiratory syndromes by 60 percent. These findings clearly address the primary research objectives and confirm the effectiveness of structured interventions in real-world settings with extremely high population density. Our findings support and extend previous research on mass collecting medicine. The direct impact of a 72 percent increase in human resources on a 54 percent improvement in case detection, combined with the strong negative correlation between preventive measures (such as education and mask distribution) and disease incidence, provides valuable operational insights for future health response design. Despite these achievements, results should be evaluated in light of limitations such as reliance on operational secondary data and considerable changes in resource deployment levels over the years. This integrated model provides a strong and transferable framework for national and international health policymakers and planners to strengthen the resilience of health systems to pandemic threats at future large-scale meetings. Future research should focus on integrating real-time surveillance systems and conducting cost-effectiveness assessments of these interventions to enhance their precision and efficiency across various environments.