How to deal with the COVID-19 outbreak while organizing a conference?

Regarding remote conferences, these have been successful. Often the motivating factor has been mitigating climate impact. For example, Yale Climate Connections wrote an article giving advice for running virtual conferences. Author Sara Peach presents three kinds of virtual conferencing, all of which have been done successfully:

• remote presentations in a traditional conference format. If people choose to not travel, they can still remote in or follow the session on social media. (Example: AGU)

• hub-based conference, where people gather in regional clusters. Each hub has some of its own content but are able to watch each other's presentations. (Example: International Conference on Music Perception and Cognition)

• an entirely remote conference, where people record and upload videos of presentations and conferencegoers have weeks to ask questions and interact. (Example: A Nearly Carbon-Neutral Conference Model)

I'll add that an entirely virtual synchronous conference has also been done successfully. One example is NACADA, the Global Community for Academic Advising, which ran a 24-hour live-streaming virtual conference.

The mixed-conference or hub models may or may not be feasible, since they would still involve people gathering in some models, and you aren't sure yet whether that will be feasible on any scale. Also, developing regional hubs would take time you don't have. Thus, if you decide to go entirely virtual, a synchronous-with-live-streaming conference and a asynchronous recorded approach have been done, and the resources linked should provide a way to start researching or planning that model. I also advise contacting your own IT department about the possibility.

Cancellations of venue, catering, etc. on your end will be entirely based on the individual companies' cancellation policies. Same with airlines for participants - each requires a certain amount of notice, and will refund or credit as per company policy. Sooner is better than later if you need to cancel arrangements for food, accommodations, etc.

The most reliable resource for info on COVID-19 at this point is the WHO, which provides situation reports daily on their website. The CDC's information is being filtered through the presidential administration, so keep that in mind when looking at White House press releases. Travel advisories are also something you'll want to keep an eye on if you're concerned. Your local health authority can also provide information about whether they are recommending cancellation of large gatherings.

If you decide to go ahead with the conference and have concerns about possible spread of coronavirus, perhaps remind attendees of the importance of handwashing and covering your cough in your opening remarks. Have hand sanitizer accessible to participants, and encourage "elbow bumps" instead of handshakes. Supplying and using facemasks is NOT being recommended at this time to the general public, so please don't go out and buy them in anticipation of the conference. This could contribute to a shortage for those who could actually need them, i.e. hospital staff.

I'm in Germany and just got an email that a trade fair and conference that was supposed to happen in the first week of April was postponed to October due to the virus.

• The email indicated that the decision was made by the organization running the fair grounds together with the local/regional health authorities.
  I'd suggest that you get into contact with your relevant health authorities.
  I'd also suggest you check the contracts for venue etc. about cancellation policies and whether it makes a difference on your side (your cancellation policies with venue etc.) whether you decide to cancel or whether the health authority tells you to cancel.

• On the participants' side: I don't think I could have gotten reimbursement for the fees that would have been due for cancelling the travel - and if probably only after months of burocratic struggle with administration.
  Update I may have been too pessimistic here: the professional society who organizes the conference now asks who had how much "leftover costs" after cancellations. They also outlined some rules (e.g. they cancelled the hotel where they had reservations for invited speakers, cancellation fees of other accomodation is probably not going to be reimbursed). end of update

  This is of course up to local rules, but if the rules around you do not reimburse if the travel had not taken place*, please cancel or postpone the meeting ASAP.
  The more so as you say it's for grad students who due to being early in their carreer did not yet have the chance to accumulate emergency funds that should be able to eat such losses.

  As a compromise (if you do not want to decide yet), you could also consider to remind people that the meeting may have to be canceled on short notice and recommend to take that into consideration for booking the travel.

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* Personally, I think this may be used as a trigger to openly discuss travel booking and reimbursement policies. IMHO there are several possible fair policies, but they should be communicated clearly and in advance: should early booking and fixed travel plans be used as they are usually cheaper? Should cancellation insurance be booked or not? What are good reasons to cancel and how does reimbursement work then?