I did actually say the quote that is going around, but the article contained vital context -- we don't know what proportion are symptomatic. Also we have only a rough estimate of what proportion of symptomatic people will have severe outcomes. 
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 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. hopkinsidd.github.io/nCoV-Sandbox/D… .
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 medrxiv.org/content/10.110… 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.)
@MauSantillana 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.
@MauSantillana Next question is what to do. While we have countable numbers of cases outside China, tracing, counting, trying to stop onward transmission one case at a time will help to buy time for those populations and on the hope that mitigating factors above hold, could maybe buy a lot
@MauSantillana Buying time matters in order to get supplies, test treatments, progress on vaccine development, and learn from the experiences of those who are currently battling this virus. This may include travel restrictons and efforts to make sure returnees don't transmit
@MauSantillana which must be done in a manner that is humane and safe for those individuals and their dependents. Equally important, as @CDCDirector has implied cnn.com/2020/02/13/hea… and PM of Singapore has stated, it may get beyond the stage of individual cases facebook.com/ChannelNewsAsi…
@MauSantillana @CDCDirector at which point will have to shift to mitigation rather than containment, and we must make sure that restrictions on travel, quarantine, and the like do not outlast their usefulness -- they are costly to individuals, families and economies and shd only be used as long as justified
@MauSantillana @CDCDirector If there is widespread community transmission driven importantly by mild or asymptomatic cases -- still a big if -- then it will be like flu and control measures targeting cases should be relaxed in favor of general social distancing to reduce contacts in general.
@MauSantillana @CDCDirector The pace of new information is overwhelming, and some of the claims in this thread have shifted from speculative to factual or very nearly so. Other points need to be modified in light of new data. See article with @BillHanage on these categories blogs.scientificamerican.com/observations/h…
@MauSantillana @CDCDirector @BillHanage Increased confidence in claims: Since I wrote this, large numbers of cases have appeared in new countries (Iran, Italy) or increased dramatically (S. Korea), increasing the evidence that this will become or already is a pandemic.
@MauSantillana @CDCDirector @BillHanage Modified in light of evolving evidence: 1) The 40-70% figure strictly speaking is only a sensible prediction for adults. Detected infections in kids to date are rare. Can happen, but rare. Still unclear if they are rarely infected or just so mild that they are rarely detected.
@MauSantillana @CDCDirector @BillHanage Until we know, safest to reserve the predictions for adults. 2) Increasing evidence that the extremely intense control measures in China have reduced transmission while they are in force. As long as such intense control measures are in place it is possible to imagine...
@MauSantillana @CDCDirector @BillHanage keeping the number infected well below 40%. I don't believe most countries (maybe not even China) can keep such controls for months or a year, the minimum time frame for potentially having a vaccine. But even more moderate ones if effective could reduce incidence considerably
@MauSantillana @CDCDirector @BillHanage So in light of @WHO's statements of the last days that these measures have been super effective, this seems the most likely way the 40% lower bound could be too high. I very much hope this is the case, but we do need to think about what controls like that for months mean.
@MauSantillana @CDCDirector @BillHanage @WHO Summary: Should have said 40-70% of adults in a situation without effective controls.
@MauSantillana @CDCDirector @BillHanage @WHO Postscript: My original quote was in the @wsj which I thought had huge circulation. Around the same time I said the same to the @TheAtlantic. The WSJ article made some ripples, but the Atlantic one went completely viral. Not what I expected.
@MauSantillana @CDCDirector @BillHanage @WHO @WSJ @TheAtlantic As the estimates of R0 are declining to sometimes below 2, I think it is appropriate to propose a lower range that accounts for more uncertainty in the true value. There is some subjectivity in deciding how much to weight diff estimates.
@MauSantillana @CDCDirector @BillHanage @WHO @WSJ @TheAtlantic Because I am now less certain of where the R0 will end up (and how it may vary geographically) I am going to revise downward the range of outcomes I consider plausible to 20%-60% of adults infected. This involves subjectivity about what range of R0 may turn out to be true.
@MauSantillana @CDCDirector @BillHanage @WHO @WSJ @TheAtlantic To preempt the critique that the earlier figures were alarmist: I update my beliefs when the available data change, as any rational person would do. The available data are pointing to a different (and better) outcome than before. So I'm updating.
