2.2 High infectivity (Why is COVID-19 so infectious?)

1. Molecular Level: SARS-CoV-2 utilizes the same cellular receptors as SARS-CoV, the human Angiotensin Converting Enzyme 2 (hACE2). Therefore, it is expected that this new virus would behave similar to SARS-CoV in terms of transmission properties, such as causing lower instead of upper respiratory tract symptoms. However, unlike SARS-CoV or MERS-CoV, epithelial cells in human airway are more suitable than standard tissue culture medium for the growth of SARS-CoV-2. 

2. Long Incubation Period & Asymptomatic Transmission: The incubation period of SARS-CoV-2 is up to 24 days and the virus is capable of human-to-human transmission among asymptomatic individuals or those displaying only mild symptoms. Unlike SARS in which most patients developed fever, a very large study involving 1,099 COVID-19 patients showed only 43.8% of them had fever at the time of admission. This renders comprehensive early identification of cases via temperature screening extremely difficult. 

3. Viral shedding is the highest early in the illness course, when patients are in the community before hospitalization.

3. Low CFR: People would be more complacent and less compliant to preventive measures.

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