At the end of August, millions of Muslims will begin the annual Hajj pilgrimage to the holy city of Mecca in Saudi Arabia. Mass gatherings like the Hajj present complex public health challenges due to the large influx of attendees from around the globe who often spend considerable time in close proximity to one another during the event, increasing the risk of disease transmission among visitors and local populations. The introduction of new pathogens and/or widespread disease transmission could compromise both the host country’s health system and those in the home countries of returning travelers who might bring diseases back with them.

Close coordination between health and event officials and prospective attendees and advanced planning are critical to mitigating the risk of disease transmission during these types of events.

Disease transmission at mass gatherings

There are many factors that could enhance disease transmission during mass gatherings, including travelers’ diverse immunological statuses, specific cultural practices that participants may engage in (e.g., mass shaving), prolonged close contact in confined spaces, and limited access to safe food and water. Additionally, mass gatherings are often attended by individuals who share similar socioeconomic characteristics, potentially forming risk groups with high susceptibility to certain diseases (e.g., groups that are largely unvaccinated for a specific disease).

The event type often contributes to the types of infections that may be transmitted as well, such as respiratory and gastrointestinal infections at events that occur in close quarters or sexually transmitted infections at events where drugs and alcohol are consumed in excess.

Specific disease risks at the Hajj

Of particular concern at the Hajj this year is cholera, which, according to the WHO, “may represent a serious risk to all pilgrims during the [Hajj] days and even after returning to their countries.” This concern is a result of the massive cholera outbreak ongoing in Saudi Arabia’s southern neighbor Yemen and others in several African countries.

Outbreaks of cholera associated with the Hajj have been recorded as far back as 1846, the most recent occurring in 1989 that affected 102 pilgrims. Investments to improve sanitation and water systems have helped to avert further cholera outbreaks.

Other disease outbreaks have also been linked to the Hajj, including multiple meningococcal outbreaks that led to the implementation of strict mandatory meningococcal vaccination requirements. However, this requirement may have had unintended consequences: one study concluded that a corresponding reduction in meningitis outbreak reports may have been more a result of the vaccination requirement serving as a disincentive for countries to report outbreaks than vaccination actually decreasing the disease burden.

Viral upper respiratory tract infections are the most commonly reported infections related to Hajj. Other potential threats include influenza, tuberculosis, measles, pertussis, and deadly coronaviruses such as MERS-CoV, which was first isolated in Jeddah, Saudi Arabia in 2012. While there have been no sustained epidemics of MERS-CoV, several hospital-associated outbreaks have demonstrated the risk of transmission in close quarters. Dedicated surveillance and diagnostic capabilities are available to monitor for MERS-CoV transmission, and Hajj pilgrimages since the emergence of MERS-CoV have “passed without any increase in travel-related MERS-CoV cases.” The incubation period for MERS can be as long as 2 weeks, so the healthcare systems of all pilgrim home countries must also be vigilant.

Saudi public health preparedness

Saudi Arabia coordinates millions of Hajj pilgrims each year, and Saudi officials have extensive experience preparing for mass gatherings. The Hajj Preventive Medicine Committee provides additional resources to support disease control at Hajj events. In addition to information on travel visas, the Saudi Ministry of Health proactively disseminates public health guidance and educational information.

Social distancing measures are nearly impossible because crowd density is overwhelming, but a number of other public health measures are implemented during the Hajj to help prevent disease transmission, ranging from access to safe food and water to vector control to free healthcare for pilgrims. Mobile isolation units are also deployed during the Hajj to supplement existing health system resources and prevent further spread of disease by separating infectious individuals from others.

Electronic surveillance systems are used to ensure the rapid detection of communicable diseases; however, due to the short duration of the Hajj—relative to disease latent periods—preventive measures are the primary focus of Saudi and Hajj health officials.

These preparedness measures have proven effective in the past, particularly during the 2003 global SARS outbreak, when Saudi authorities “recognized the threat early and instituted a variety of control measures, including restrictions of arrivals from countries with recognized outbreaks, home quarantine, and rapid testing.”

What to expect this year

The 2016 Hajj saw more than 1.8 million visitors (1.3 million from outside Saudi Arabia), and similar numbers are likely to participate this year. Identifying infectious disease threats to mass gatherings—whether they are religious pilgrimages like the Hajj, sporting events like the World Cup or the Olympics, or others—and implementing effective public health interventions are paramount to preventing infectious disease importation or exportation.

While this seems like a daunting task given the sheer number of participants at these types of events, coordinated efforts between health officials in the host country, travelers’ origin countries, and event officials to implement appropriate disease surveillance systems, preventive interventions, response capabilities, and education campaigns can mitigate the effects of local or global outbreaks or even stop them before they begin.