Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19 | NEJM

https://www.nejm.org/doi/full/10.1056/nejme2009758

…Within 8 months, SARS was controlled …. Within 5 months, SARS-CoV-2 has infected more than 2.6 million people ….

What explains these differences in transmission and spread? A key factor
in the transmissibility of Covid-19 is the high level of SARS-CoV-2
shedding in the upper respiratory tract,1
even among presymptomatic patients, which distinguishes it from SARS-CoV-1, where replication occurs mainly in the lower respiratory
tract.2
Viral loads with SARS-CoV-1, which are associated with symptom onset, peak a median of 5 days later than viral loads with SARS-CoV-2, which makes symptom-based detection of infection more effective in the case of
SARS CoV-1.3
With influenza, persons with asymptomatic disease generally have lower
quantitative viral loads in secretions from the upper respiratory tract
than from the lower respiratory tract and a shorter duration of viral
shedding than persons with symptoms,4 which decreases the risk of transmission from paucisymptomatic persons (i.e., those with few symptoms).

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