Every now and then a study deserves a stand-alone summary. There is so much that is not clear about CV-19 transmission. This study was a survey of research on transmission. (Annals Article) The authors conclude that the respiratory route of transmission in the only one clearly proven, that while fecal and other routes have been hypothesized, there is evidence of only one possible case of such transmission. Transmission among animals has been shown, but from animals to humans is generally not proven, with the exception of minks. Droplets also appear to be the primary method of respiratory transmission, although aerosol transmission is likely. While there is some evidence of virus survivability on a surface, the amount of viable virus diminishes rapidly, as it does in aerosols. Proximity appears to be a key factor in transmission, which also suggests droplets as the primary mechanism. Ventilation may also play a role in the likelihood of transmission.
The viral load is highest in the upper respiratory tract in the initial stages of infection and in the lower respiratory tract in the later stages of disease. Viral load is likely correlated with disease severity. It is unclear what the necessary viral dose is for infection. There is wide variation in susceptibility to infection, which rises with age. There is also wide variation in transmission by infected persons, with most not transmitting and a few transmitting to many people. Studies suggest that 10% of infected persons may be responsible for 80% of transmission. Infectiousness peaks on the day before symptoms appear and declines within a week. There is no proven case of transmission more than a week after symptom appearance, even in those with severe disease. Studies suggest there is no viable virus in samples tested by PCR with a cycle number of over 24.
Worth a close read to understand what we do and don’t know about transmission at this point.