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.
The Healthy Skeptic is a website about the health care system, and is written by Kevin Roche, who has many years of experience working in the health industry. Mr. Roche is available to assist health care companies through consulting arrangements through Roche Consulting, LLC and may be reached at 
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Kevin,
Perhaps I’m misunderstanding the clinical differentiation between droplets and aerosols. In the paper (1st para in Respiratory Transmission” section) the authors state, “however, this dichotomization may be an oversimplification, and distinguishing droplet and aerosol transmission is difficult in clinical settings.” The further confusion is their definitive conclusion “That proximity so clearly increases risk for infection suggests that classic droplet transmission is more important than aerosol transmission (51).” The two statements seem to me to be at odds; aerosol transmission is greatly increased by proximity.
My disagreement is with the latter statement and with your statement, “Proximity appears to be a key factor in transmission, which also suggests droplets as the primary mechanism.” Were the primary transmission factor droplets, it would seem to argue against, for example, the South Korea call center, among others, and should mean that masking studies would have some level of consistency in effectiveness.
Yes, this paper, due to the intensity of its topics, was worth the extra review, but too many of their conclusions seem arbitrary or counter to other findings. Too many statements seem to be in search of a prejudicial viewpoint.