Transmission of SARS-CoV-2: Review of Viral, Host, Environme
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This systematic review of the transmission of SARS-CoV-2 addresses the environmental viability of the virus, viral and host transmission factors, modes of transmission, period of infectiousness, and population transmission dynamics.

The following are 10 key points summarizing its findings:

1. In the experimental setting, viable virus was isolated for up to 3 hours from aerosols and up to 72 hours from various surfaces; the longest reported viability was on plastics and stainless steel, with half-lives around 6 hours.

2. The virus is highly stable at low temperatures but sensitive to heat, with inactivation of the virus in 5 minutes at 70°C (158°F). Disinfectants also prevent culture of SARS-CoV-2.

3. In real-world settings, SARS-CoV-2 RNA but not live virus has been detected in high-touch surfaces. Levels are, however, much lower than those in the nasopharynx of infected individuals. Isolation of the virus from air particles is rare.

4. Viral load is highest in the upper respiratory tract (nasopharynx and oropharynx) early in disease and then increases in the lower respiratory tract (sputum).

5. Susceptibility to SARS-CoV-2 infection increases with age; children less than 10 years (4-7%) are around half as susceptible as adults (14-17%). Household contacts are more likely to transmit the virus.

6. SARS-CoV-2 bearing the D614G mutation is thought to increase the transmissibility of the virus through improving binding of the S protein to angiotensin-converting enzyme 2 (ACE2).

7. The dominant route of transmission of SARS-CoV-2 is respiratory. Direct contact or fomite transmission is uncommon. In-utero transmission is likely rare. Sexual, fecal–oral, and bloodborne transmission are theorized but have not been documented. There are no confirmed cases of transmission through breast-feeding or from domestic pets to humans.

8. Proximity and ventilation are key determinants of transmission, which is more likely in densely packed and poorly ventilated areas. Masking and excellent hand hygiene decrease transmission of the virus in both health care and community settings.

9. SARS-CoV-2 transmission peaks 1 day prior to onset of symptoms. Transmissibility correlates with severity of symptoms, although asymptomatic individuals can also transmit the virus, albeit at a lower rate. The period of infectiousness is typically much shorter (<9 days of symptoms) than the duration of detectable RNA shedding.

10. SARS-CoV-2 transmission is highly overdispersed; i.e., 80% of infections arise from 10% of index cases (superspreaders). Host factors that contribute to superspreading events remain unknown.

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