Mass Gatherings

CDC Yellow Book 2024

Travel for Work & Other Reasons

Author(s): Joanna Gaines, Kristina Angelo

Mass gatherings are typically defined as large numbers of people (>1,000) at a specific location, for a specific purpose. Practically speaking, a mass gathering can be any assembly of people large enough to strain local resources. Travelers to mass gatherings face unique risks because these events are associated with environmental hazards, challenging security situations, and increased opportunity for infectious disease transmission due to the influx of attendees, crowding, and poor hygiene from temporary food and sanitation facilities. Although the coronavirus disease 2019 (COVID-19) pandemic caused the cancellation or postponement of numerous mass gatherings, a growing number are being held, with mixed public health consequences.

Mass Gathering Characteristics

International travelers and their medical providers should understand the characteristics of mass gatherings. Some can be spontaneous (e.g., political protests); others are planned events. Some mass gatherings regularly occur at different locations (e.g., the Olympic Games, the Fédération Internationale de Football Association [FIFA] World Cup); other gatherings recur in the same location (e.g., Hajj, the annual Islamic pilgrimage to Mecca [see Sec. 10, Part 1, Ch. 2, Saudi Arabia: Hajj & Umrah Pilgrimages]). Table 9-04 provides a brief representative list of mass gatherings, including type (religious observance, sporting event, or art and music festival), location, and typical attendance numbers. Most mass gatherings can be described effectively in terms of their activities, capacity, duration, location, participants, purpose, size, timing, and venue (see Table 9-05).

Table 9-04 Examples of international mass gathering events




Saudi Arabia

2.5 million


FIFA World Cup 2022


3 million

Cultural (arts/music)


Brazil (also worldwide)

7 million


EXPO 2023


>5 million

Table 9-05 Mass gatherings: characteristic features & potential risks



Some activities can be risky or strenuous (e.g., walking long distances in extreme temperatures) or could involve alcohol or drug use.


Hosts differ in their ability to detect, respond to, and prevent public health emergencies. 

Understanding health outcomes previously associated with a recurring mass gathering can help travelers prepare for future events. 

Security arrangements vary.


The longer an event lasts, the more likely local resources will be depleted and become strained.


Environment and infrastructure affect health and safety of events; some host countries and cities have better natural or engineered resources to handle large numbers of people than others.


Attendees can represent a unique demographic (e.g., religious, political groups), or vary by gender or age (e.g., older adults attempting to complete a religious pilgrimage toward the end of their life).


Mass gatherings can be political, religious, social, or athletic events. 

The purpose of an event can determine the activities and affect the mood of participants.


The density of crowds, not just the number of attendees, contribute to health and safety risks. 

More densely packed crowds can facilitate disease spread or induce riots or crowd crush disasters.


Mass gatherings and local capacity are affected by the timing of an event. 

Season / weather can influence the number in attendance which affects the host’s ability to organize a safe mass gathering.


Indoor versus outdoor events create different sets of challenges for mass gathering organizers. 

Food, water, housing, and sanitation, can be of varying quality.


Mass Gathering–Associated Health Concerns

Attendance at a mass gathering can exacerbate a traveler’s existing medical conditions. Emergency medical services often are involved in preparations for gatherings, and are usually equipped to address acute medical conditions (e.g., asthma, gastrointestinal issues, injuries, myocardial infarction). Onsite healthcare providers are usually capable of handling gathering-associated conditions (e.g., dehydration, heat exhaustion, hypothermia, sunburn).

Catastrophic Incidents

Catastrophic incidents are of particular concern during mass gatherings, especially with extremely dense crowds. Numerous casualties have occurred at mass gatherings due to poor crowd management, structural collapses, fires, and violence. Crush injuries and death can result from crowding and stampedes. At the 2015 Hajj pilgrimage in Saudi Arabia, for example, thousands of pilgrims died in a stampede; in 2021, dozens of pilgrims, including children, were killed during a stampede at an annual Lag BaOmer festival in Israel.

Ensuring personal safety during mass gatherings is necessary. Travelers should remain aware of their surroundings. Although the risk for large-scale incidents (e.g., terror attacks) are low, they are impossible to predict or eliminate (see Sec. 4, Ch. 11, Safety & Security Overseas).

Infectious Diseases

Mass gathering attendees are at risk for infections, including vaccine-preventable illnesses (e.g., COVID-19, influenza, pneumococcus). Past mass gatherings have been associated with outbreaks of influenza, meningococcal disease, and norovirus. Mass gatherings also have implications for global health security.

Travelers who import infectious diseases to mass gathering host sites can infect both their fellow attendees and local organizers who, in turn, can become sources of infection to others. In this way (and depending on routes of transmission, incubation periods, and other disease-specific factors) concentrations of people attending mass gatherings facilitate the amplification of a disease. Participants can then export the illness internationally to destinations other than the host location. For emerging diseases, little might be known at first about all the various routes of transmission or consequences of infection. When Zika initially emerged in 2015 in Brazil, for example, shortly before the country hosted the Olympic and Paralympic Games, the potential for sexual and vertical transmission of the virus was unknown.

Coronavirus Disease 2019

COVID-19 poses a unique risk for both travelers to mass gatherings and for the countries hosting mass gathering events, due to the influx of international attendees, varied country-specific immunization practices and vaccine access, and difficulty enforcing mask use or physical distancing. Logistics related to staff safety, visitor safety, site circulation (e.g., queuing), event location (e.g., indoor vs. outdoor), commerce (e.g., food and beverage venues), and sanitation (e.g., availability, location, and number handwashing stations) are among the issues to be considered when planning or visiting an event during the pandemic.

COVID-19 has impacted recommendations and requirements for all international travelers, including mass gathering attendees. Some countries require travelers to demonstrate proof of vaccination (including ≥1 booster dose) against COVID-19, prior to entry. Others may require mandatory quarantine periods for those not vaccinated. Travelers should check with local health authorities at their intended destination to ensure they are aware of the most current requirements.

Clinician Guidance

Risk Assessment

Activities & Itineraries

Ask travelers about their activities and itineraries. Verify a traveler’s itinerary to identify risks at the destination, in addition to those associated with the event itself. Travelers might add side trips or extend travel beyond the mass gathering.

Patient Characteristics

Consider the patient’s unique characteristics. Chronic health conditions can be exacerbated while participating in a mass gathering. Counsel patients on the importance of having adequate supplies of medication for the duration of their trip, and documentation for any prescriptions.

Risk Mitigation


Encourage vaccination. Ensure that travelers have all appropriate pretravel vaccinations, including routine and required vaccines, and are up to date with their vaccinations against COVID-19.


Identify requirements for mass gathering attendees beyond those required for entry to a country. For example, whereas Saudi Arabia mandates that all participants in the Hajj be vaccinated against meningococcal disease, this requirement does not apply to other travelers visiting the country.


In addition to any host-country requirements, some destinations and venues can have additional recommendations for mass gathering attendees based on public health concerns (e.g., demonstrating proof of COVID-19 vaccination). Be prepared to identify and provide needed pretravel health services based on host-site recommendations.

Provide Education & Guidance to Travelers

Educate travelers on preventive measures, including regular application of sunscreen and insect repellent or advice on how to choose safe food and water from vendors. Emphasize the importance of regular handwashing with soap and water and the use of hand sanitizer with ≥60% alcohol content when sanitation facilities are not available. Box 9-10 is a checklist for travelers to use as they plan to attend a mass gathering.

Stay Current

The Centers for Disease Control and Prevention regularly updates its Travelers’ Health website with Travel Health Notices (notifications of disease outbreaks in countries around the world) and information on select mass gatherings (e.g., Hajj, the Olympic Games).

Box 9-10 Mass gathering events: a planning checklist for travelers


  • In addition to the entry requirements of the host nation, become informed about all requirements for participating in the mass gathering (e.g., medical tests, proof of vaccination, use of smartphone applications).


  • Make an appointment to see a travel medicine specialist at least 4–6 weeks before travel—this should allow enough time for you to receive most of your necessary vaccinations.
  • Discuss your itinerary and any planned activities with the provider—this will allow your provider to make more accurate recommendations to ensure your health and safety.
  • Discuss your medical history with the travel medicine specialist—some travel medicines might not be safe to take with medicines you take regularly.
  • If a travel medicine provider is not locally available, a primary care provider should be able to ensure you have adequate vaccinations and the health information necessary.


  • Work with your regular health care provider to make sure existing medical conditions are well controlled before you leave.
  • Ensure you have an adequate supply of all your regular prescription medicines prior to departure.


  • STEP provides travelers with notifications (e.g., travel warnings, travel alerts, other destination-specific information).
  • STEP also makes sure that the State Department can find you if you experience serious legal, medical, or financial difficulties overseas.
  • In case of emergency at home, STEP can help friends and family contact you.


  • Learn more about health and safety issues at specific destinations.
  • Find out if CDC has posted any Travel Health Notices for your destination or mass gathering event.


The following authors contributed to the previous version of this chapter: Joanna Gaines, Gary W. Brunette

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