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Marc Lipsitch
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.
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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)
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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. .
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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
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Marc Lipsitch Feb 14
Replying to @mlipsitch
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.
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Marc Lipsitch Feb 14
Replying to @MauSantillana
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 et al.)
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Marc Lipsitch Feb 14
Replying to @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.
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Marc Lipsitch Feb 14
Replying to @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
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Marc Lipsitch Feb 14
Replying to @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
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Marc Lipsitch Feb 14
which must be done in a manner that is humane and safe for those individuals and their dependents. Equally important, as has implied and PM of Singapore has stated, it may get beyond the stage of individual cases
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Marc Lipsitch Feb 14
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
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Marc Lipsitch Feb 14
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.
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