Here's what I think I understand about basic epidemiology numbers: -We can measure the doubling time simply, by counting cases. -It takes significant effort to evaluate the serial interval (i.e. the time it takes for someone to contaminate others). (1/9)
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-We can't measure R (the effective reproduction numer) directly, it has to be derived from the two numbers above. -Finally, R0 (the basic reproduction number) comes from knowing R and the proportion of the population that's already immune. In the early days, R0 = R. (2/9)
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Now, here's what nasty about covid-19: we have no idea what the serial interval is. Some say it is about 4 days: https://wwwnc.cdc.gov/eid/article/26/6/20-0357_article … Others assume 6-9 days: https://wwwnc.cdc.gov/eid/article/26/7/20-0282_article … (3/9)
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In turn, that means that we don't know what R0 is. The former number implies R0=1.32: https://www.jwatch.org/na51171/2020/03/27/serial-interval-covid-19 … while the latter number finds R0=5.7. (4/9)
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That inconsistency has to be a horrible situation for government officials: if the serial interval and R0 are low, contact tracing is hard (because you might have to trace multiple generations of contacts) and distancing is effective (because it's easy to lower R0 below 1) (5/9)
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On the other hand, if those numbers are high, contact tracing becomes easier (fewer generations to trace) but distancing needs to be extreme (because getting R0 below 1 requires massive efforts). (6/9)
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Looking at numbers in European countries, which I think have primarily relied on distancing, R0 is below 1, but not by much, such that the halving time might be measured in weeks (eyeballing from various charts on https://www.worldometers.info/coronavirus/ ) (7/9)
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China, with a harder lockdown, might have gotten their halving time down to maybe 4 days (eyeballed). South Korea might have seen similar numbers, by combining distancing with contact tracing. (8/9)
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I don't think Western countries are out of the woods. Our social distancing barely gets R0 below 1. I think that we'll see longer distancing orders, and that we'll need to add contact tracing in order to accelerate the decline of the epidemic. (9/9)
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When R varies with time (as is the case with dynamic interventions such as social distancing) I think it's usually written R_t. See https://wwwnc.cdc.gov/eid/article/25/1/17-1901_article …
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