A bio-secure bubble, also known as a bubble,[1] or hub city,[2][3] was a hosting arrangement for sporting events that emerged during the COVID-19 pandemic, under which events were held at a centralized site, often behind closed doors, with strict quarantine and safety protocols in order to prevent the spread of COVID-19. A bubble was established for a single sports season, tournament, or for an ongoing series of events, allowing them to still be held and made available to broadcast audiences.