A careful study of 468 infector-infected pairs found that 12% involved transmission *before* the infector was symptomatic. #mismatch #quarantineloophole https://www.medrxiv.org/content/10.1101/2020.02.19.20025452v2.full.pdf … 17/
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Examination of 124 Wuhan cases with clear contact history showed incubation period of 5 days (range: 1-11 days). It found that 73% of secondary cases were infected *before* onset of symptoms in first case. It estimated the mismatch period to be 3 days. https://www.medrxiv.org/content/10.1101/2020.03.06.20031955v1.full.pdf … 18/
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A fine analysis of cases from China (N=135) & Singapore (N=93) found incubation was ~7-9 days. And there was pre-symptomatic transmission, with infection occurring on average 2.9 days (China) & 2.6 days (Singapore) before symptom onset in the infector https://www.medrxiv.org/content/10.1101/2020.03.03.20029983v1.full.pdf … 19/pic.twitter.com/XtNk0iUPpx
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Another analysis looking at 28 pairs of individuals suggests that "a substantial proportion of secondary transmission may occur prior to illness onset.” https://www.medrxiv.org/content/10.1101/2020.02.03.20019497v2 … 20/
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Study of 52 Chinese patients found incubation period to average 5 days, w typical range of 2-14 days. Such analyses yield the widespread recommendation of 14 day quarantine period (so as to be sure person is not infected, whether symptomatic or not). https://www.medrxiv.org/content/10.1101/2020.01.26.20018754v2.full.pdf … 21/
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A further wrinkle is that these numbers are averages. Even if the latent period were not shorter than the incubation period on average, it could still be so for some patients. Indeed, this may be one of the factors that can make some individuals super-spreaders. |
@salathe 22/pic.twitter.com/IDxoTJys4G
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The likely existence of asymptomatic transmission of SARS-CoV-2 causing COVID19 does NOT mean we should not bother to test patients (e.g., to see if they are febrile) and ask them to self-isolate. Isolating carriers and patients is still essential! 23/
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It is wise to detect & sequester patients, as is widely done (China, Korea, Singapore) by offering widespread free testing using real-time (rRT-PCR) detection of the SARS-CoV-2 virus (detecting viral RNA) https://www.cdc.gov/coronavirus/2019-ncov/lab/testing-laboratories.html … and (much easier for screening) fever checks. 24/
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In sum, on average, COVID19 patients take about 7 days from exposure to SARS-CoV-2 to show symptoms (typical range 2-14 days), and a meaningful percentage of carriers can spread the disease for 2-4 days before they are symptomatic. We have our work cut out. 25/
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Replying to @NAChristakis
This is also likely why places with the supply of masks and the culture of universal mask wearing are doing so much better compared to places where we have such a shortage that that we cannot advocate for masks for ordinary people—even healthcare workers don’t have enough.
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It’s pretty clear telling people to wear masks when they know they’re sick doesn’t work with asymptomatic transmission. Plus dearth of testing! Let alone the stigma of having masks signal sick people, thus discouraging use.
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