What factors should organizers and health authorities look at when assessing whether the risks are acceptable or not?

For countries not currently known to be experiencing community transmission of COVID-19, the priority consideration will be whether the planned mass gathering event substantially increases the risk of the virus entering the country and becoming established, as well as the risk for participants to importing infection back to their home country and further increasing global spread. In making this assessment, the organizers and their national or local health authorities should recognize that the risk of imported cases of COVID-19 is naturally linked to international travel. They should also recognize that it is neither realistic or desirable to aim for zero risk. When organizers and health authorities are determining whether to hold a mass gathering, they should determine what is an acceptable risk and what additional measures should be implemented to mitigate the risks.

For countries where COVID-19 has already started to spread in the community, key consideration will be:

aiming at containing or at least slowing down the spread of the virus in the local community/country. preventing participants from other countries being infected with COVID-19
In each case the risk should be considered in the context of the known features of COVID-19, its severity, its transmissibility and the effectiveness of measures to prevent or reduce transmission. The strain already placed on the local health system in responding to COVID-19 outbreak(s), and the additional strain the mass gathering might place on the system also need to be taken into account.