Yeah 7 days was intended to be a reasonable conservative guess. My actual guess is closer to 5.5. As you point out there are testing artifacts that point in both directions. Within china, test shortages, outside of china, slower testing roll out. I’m not an epi expert but I think the gold standard here would be to do something like time-series immune surveillance, where you randomly sample a large group of people from a pop and test them for an antibody reaction and/ or viral RNA, then do the same at intervals later. My guess is this is challenging because of the number of samples required to detect in most places, but maybe if you did this somewhere like italy you could pull it off (you get the population abundance as well).
Its also the case that this isn’t a fixed number, and you expect it to vary from population to population based on fraction of asymptomatic cases, social distancing, pop density etc. So I’m not sure we’ll get a better number than 2-8 days in the short term, which is disconcerting given how big of a difference it makes to risk forecasts.
I’d love to hear from anyone with more epi expertise!
Thanks for writing this!
I’d be interested in pointers on how to interpret all the evidence on this:
until Jan 4: (Li et al) find 7.4 days
Jan 16–Jan 30: (Cheng & Shan) find ~1.8 days in China, before quarantine measures start kicking in.
Jan 20–Feb 6: (Muniz-Rodriguez et al) find 2.5 for Hubei [95%: 2.4–2.7], and other provinces ranging from 1.5 to 3.0 (with much wider error bars).
Eyeballing the most recent charts:
outside China looks like ~4–5 days
South Korea and Italy look shorter (~2-3 days?)
I’ve also seen it suggested that the outside-China growth might be inflated due to ‘catch up’ from slow roll-out of testing.
Altogether, what is our best guess, and what evidence should we be looking out?
Yeah 7 days was intended to be a reasonable conservative guess. My actual guess is closer to 5.5. As you point out there are testing artifacts that point in both directions. Within china, test shortages, outside of china, slower testing roll out. I’m not an epi expert but I think the gold standard here would be to do something like time-series immune surveillance, where you randomly sample a large group of people from a pop and test them for an antibody reaction and/ or viral RNA, then do the same at intervals later. My guess is this is challenging because of the number of samples required to detect in most places, but maybe if you did this somewhere like italy you could pull it off (you get the population abundance as well).
Its also the case that this isn’t a fixed number, and you expect it to vary from population to population based on fraction of asymptomatic cases, social distancing, pop density etc. So I’m not sure we’ll get a better number than 2-8 days in the short term, which is disconcerting given how big of a difference it makes to risk forecasts.
I’d love to hear from anyone with more epi expertise!
Related- possibly first use of immune surveillance: https://www.sciencemag.org/news/2020/02/singapore-claims-first-use-antibody-test-track-coronavirus-infections