"Why do I think a pandemic is likely? The infection is in many parts of China and many countries in the world, with meaningful numbers of secondary transmissions. The scale is much larger than SARS for example (where the US had many introductions and no known onward transmission)



Why do I think 40-70% infected? Simple math models with oversimple assumptions would predict far more than that given the R0 estimates in the 2-3 range (80-90%). Making more realistic assumptions about mixing, perhaps a little help from seasonality, brings the numbers down.



Pandemic flu in 1968 was estimated to _symptomatically_ infect 40% of the population, and in 1918 30%. Those likely had R0 less than COVID-19. Below is from https://stacks.cdc.gov/view/cdc/11425





What could make this scenario not happen? 1) conditions in Wuhan could be so different in some fundamental way from elsewhere that we are mistaken in expecting further outbreaks to have basic aspects in common. No reason I know of to think that but a formal possibility



2) There could be a higher degree of superspreading than has been appreciated ("dispersion in R0") which could mean that many locations outside Wuhan could "get lucky" and escape major onward transmission. https://hopkinsidd.github.io/nCoV-Sandbox/DispersionExploration.html



This seems the most likely way a pandemic might be averted, but given the number of countries infected and likely missed imports in many of them that seems a lot to hope for



3) Control measures could be extremely effective in locations that have had time to prepare. Maybe in a few, but seems unlikely that is the case in all, especially countries with stretched health systems.



4) Seasonal factors could be much more powerful at reducing transmission than we currently expect. That doesn't help the Southern hemisphere, and is not consistent with behavior in China (preprint in queue from @MauSantillana et al.)



So that's my reasoning. It is as tight as I can make it and is an effort to use the science as we have it to make our best estimate about the future. Predictions can be wrong and I very much hope this is, but better to be prepared."