Mass-gathering epidemiology is an emerging discipline in applied public health.1 High-profile mass gatherings include major sporting events (eg, the Olympics, the FIFA World Cup [Fedération Internationale Football Association]), religious events (eg, the Hajj, World Youth Day), cultural festivals (eg, Glastonbury Music Festival), and US National Special Security Events (eg, political conventions), among other locally defined events. These events may impose short-term pressures on local and regional public health infrastructure. In accordance with the International Health Regulations, the World Health Organization offers guidance for public health planning, surveillance, and response during mass gatherings.2,3

Public health risks associated with mass gatherings are well documented and encompass a variety of focus areas, from environmental health hazards to infectious diseases.4,5 Because of the diverse nature of mass gatherings, different factors contribute to the health and safety risks for participants. A 2002 review of the mass-gathering medical literature categorized different variables and their possible causal relationships to health outcomes. These variables included weather, attendance, duration of event, location of event, event type, crowd mood, alcohol or drug use, crowd density, and age of attendees.5 Although infectious disease outbreaks and injury clusters have been reported during mass gatherings,1,6–9 the large number of annual events held worldwide without reports of adverse events suggests that these occurrences are relatively rare—though publication bias may have led to some underestimation. Nonetheless, a local public health agency must be prepared to enhance its surveillance capacity to detect and investigate an outbreak, mass exposure, or injury cluster that could damage the credibility of the event or exert a substantial human or economic impact.

Because of public health, political, and economic influences, mass gatherings will inherently require enhancing existing surveillance capacity.1,10–12 Enhancing surveillance implies increasing the timeliness and accuracy of existing surveillance capacities and/or implementing novel, supplemental, active surveillance systems. Enhancing surveillance for mass gatherings requires not only a thorough risk assessment to inform what health outcomes or exposures to prioritize12 but also adequate existing capacity to receive and analyze information rapidly.1 Most commonly, enhanced surveillance for mass gatherings begins with reportable disease surveillance (ie, mandated disease reporting, laboratory-based surveillance) and typically involves accelerating the speed of disease reporting or electronic data transmission.13 Although some enhancements may be beneficial only for the duration of the event, most should provide a permanent benefit for the host country’s public health infrastructure.12

Professor Memish, an expert in mass-gathering medicine, wrote that “there is an urgent need to address health issues related to mass gatherings, such as the need for systems and adequate capacity to be in place to receive and analyze information rapidly from surveillance.”1 However, in determining where and if an electronic syndromic surveillance system fits into an enhanced surveillance plan for a high-profile mass gathering, Professor Reingold is equally prudent: “If syndromic surveillance is the answer, what is the question?”14 In other words, given limited public health resources, surveillance requires identifying actionable public health outcomes that justify the cost of the system. In the mass-gathering setting, enhanced surveillance necessitates asking appropriate questions that can be accurately measured by the surveillance system and lead to a timely public health intervention.

Electronic syndromic surveillance systems that receive and analyze near real-time data from hospital emergency departments, emergency medical services, poison control centers, or other similar sources have increasingly become common tools for enhanced surveillance and may provide timely and actionable information during high-profile mass gatherings.11,15,16 However, a number of epidemiologic infrastructure requirements should be met before these systems are considered useful to a local jurisdiction’s enhanced surveillance portfolio for a mass gathering. If the following 4 requirements can be met, it is increasingly likely that, if available, a syndromic surveillance system may be useful during a mass gathering.

Plan for a Sustainable Investment To be most effective, a syndromic surveillance system must be well established before the event. That is, the system should already be part of routine surveillance practice or implemented well in advance of the event—the data are flowing, the technical errors have been worked out, and the outputs are being routinely interpreted in local context. In contrast, “drop-in” syndromic surveillance—an active, syndrome-based surveillance method designed specifically for an event—is expensive, labor intensive, and unsustainable and often fails to account for accurate baseline information or local context.17 Drop-in syndromic surveillance has historically used paper-based data collection methods,18 but new field-based digital collection techniques (eg, wireless and mobile technologies) and internet-based (eg, social media, mass media) technologies may offer cost-effective solutions for short-term event-based surveillance. However, these new digital methods should undergo further validation and cost-benefit analysis before they can supplement traditional surveillance.19–22 Nonetheless, drop-in surveillance is designed to be a short-term solution without long-term value. With limited public health resources, sustainability of any surveillance system is essential. A mass gathering can serve as an opportunity to enhance existing disease surveillance and/or invest in new surveillance with the intent to sustain the system for long-term use. During the 2006 FIFA World Cup, German health officials focused on improving mandatory disease reporting and chose not to introduce syndromic surveillance.13 Sustainable enhancements made to their mandatory disease-reporting system strengthened detection capabilities and improved overall timeliness. For the 2004 Democratic National Convention, the Boston Public Health Commission invested approximately $500 000 to develop and implement a syndromic surveillance system before the event.23 The system was integrated into routine surveillance practice and has demonstrated utility over time.24 Likewise, a comprehensive syndromic surveillance system enhanced for the 2012 London Olympics served as a public health legacy for the United Kingdom’s Health Protection Agency.25 Regardless of the methods used to enhance surveillance, investments should lead to sustainable improvements in surveillance infrastructure.

Enhance Traditional Surveillance, Then Integrate New Data Sources It is also important that syndromic-based surveillance systems supplement more specific disease- or exposure-based surveillance systems, such as reportable disease surveillance based on electronic laboratory reporting or standard environmental health surveillance (eg, toxic substances surveillance; food, lodging, and sanitary inspections). Enhancements to traditional reportable disease surveillance include increasing the completeness and timeliness of disease reports (as well as the capacity to receive reports) and increasing electronic data exchange. Syndromic surveillance can complement traditional surveillance by integrating interpreted surveillance information in an epidemiologic “dashboard” (a visual display of data and information) or “common operating picture.” Bear in mind, though, that situation awareness, rumor control, or epidemic intelligence cannot be addressed by a single surveillance system but require combined surveillance and information-gathering efforts to provide context and internal validation, of which syndromic surveillance may serve as one of many inputs.26

Develop Actionable Questions As in the development of any surveillance system, using syndromic surveillance during mass gatherings requires asking appropriate questions of the data before the event. Those questions should result from a risk assessment, which must be performed before and during the event to determine what public health-related outcomes are most likely to occur and whether these outcomes can be addressed through syndromic surveillance.12 Past experience with syndromic surveillance during mass gatherings has demonstrated that injury-related outcomes, such as heat-related illness, motor vehicle accidents, and injuries, are more accurately measured (ie, higher signal-to-noise ratio, greater sensitivity and specificity, more likely to occur) than high-consequence infectious disease cases, outbreaks, or incidents of bioterrorism.6,11,15,27–29 Unlike other surveillance systems, however, syndromic surveillance is particularly flexible to address new questions as they present, which makes these systems useful for unpredictable outcomes.30,31 If all emergency department visits, poison control center calls, or emergency medical service runs are routinely collected, a new case definition for an illness or injury can be easily added and quickly analyzed. For example, crushing injuries could be added to an established syndrome-based system to more accurately assess the impact of a stampede at a mass-gathering event32 or mental health outcomes (eg, acute posttraumatic stress) after terrorism events.33

Disseminate Accurate Interpretation Because syndromic surveillance tends to be based on aggregate trends with varying sensitivities and specificities, it requires more interpretation for untrained stakeholders. Aberration detection methods flag trends or patterns and can use keywords or natural-language processes to identify possible cases that require additional epidemiologic investigation. Occasionally, syndromic surveillance can detect public health threats or support an epidemiologic investigation of an outbreak.8,9 Often, however, syndromic surveillance does not detect any true public health threats, simply because they are rare, they are identified by traditional surveillance first, or they are missed by the system.34–36 Perhaps the most concerning limitation for identifying actionable outcomes via syndromic surveillance is the duration of the event. The shorter the event (eg, <3 days), the less likely an aberrant flag can be identified, investigated, and intervened before the event ends. Additionally, mass gatherings can impart unusual statistical abnormalities to a syndromic surveillance data stream. For example, baseline data can quickly be affected by the residential population’s decision to leave town or avoid health care during the event because of disruptions in normal routines, such as traffic patterns. These limitations require careful interpretation by mass-gathering stakeholders.

Conclusion There are many ways to enhance surveillance for mass gatherings. Electronic syndromic surveillance is only one of these. Despite its limitations and operational costs, syndromic surveillance may provide useful information to local health departments during a mass gathering. However, a preexisting infrastructure to support electronic data transfer, a capacity to quickly receive and interpret data, a thorough understanding of how to interpret and share these data with external stakeholders, and a plan to sustain any new surveillance enhancements are necessary before electronic syndromic surveillance can be used effectively during mass gatherings.

Acknowledgments The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding

The authors received no financial support for the research, authorship, and/or publication of this article.