High profile international sporting events such as the Olympics or World Cups as well as international religious events such as the Hajj count as mass gatherings. However, lower profile conferences and events can also meet WHO’s definition of a mass gathering.
An event counts as a “mass gatherings” if the number of people it brings together is so large that it has the potential to strain the planning and response resources of the health system in the community where it takes place. You need to consider the location and duration of the event as well as the number of participants.
For example, if the event takes place over several days in a small island state where the capacity of the health system is quite limited then even an event with just a few thousand participants could place a big strain on the health system and then be considered a “mass gathering” event. Conversely, if the event is held in a big city in a country with a large, well-resourced health system and lasts just a few hours, the event may not constitute a “mass gathering” event.
(Sources: WHO – World Health Organization Website)