[M T]

One thing to keep in mind is that the probability of infection is related to the number of viable viral particles that are transmitted to an infectable surface (I assume a mucous membrane) and how direct the transmission is.

A number of misunderstandings there, based on the medical articles I'm reading. 

Main mode of transmission is by inhaling droplets from the infected person's breathing.   Coughing/Sneezing is not needed (as aKidDoc said earlier).
In 2003's SARS-COV-1 days, at least one study in healthcare situations indicated failure to wear a mask resulted in more illnesses than failure to wear gloves.

There is no need for a very sick person to do the breathing.  The seemingly healthy guy at your door dropping off groceries can give you a dose.  The virus shedding is maximal early in the disease.   The evidence has been that a person is infections between 2 to 3 days before the first symptom.

There are cases where person A infects person B, and person B gets the symptoms well before person A.
The serial interval (time that B shows symptoms relative to A) is in the 4-5 day range, which is shorter than the incubation period.

---- details

In this Mar 13 preprint on serial interval (time between a series of infections),

Notably, 59 of the 468 reports indicate that the infectee developed symptoms earlier than the infector. Thus, pre-symptomatic transmission may be occurring, i.e., infected persons may be infectious before their symptoms appear.

the serial interval was as low as -11 days to as high as 20 days, with a mean of 4 days and a standard deviation of 4.75 days.

Wow!  I had seen indications of very short serial intervals (the Mar. 6 report that lead this particular thread had serial interval as low as 2+ days (ie, 2 days after infection of person A, person A infected person B))   Apparently, the variability in onset of symptoms (incubation) is large enough to allow B's symptoms to show up before A's symptoms.  


By the way, links to a number of papers can be found here.

China's 1099 patient study, the 291 for which documented exposure existed, had a mean incubation 4 days, with 25 percentile at 2 days or less, 75 percentile at 7 days.  At hospital admission (all positive tests in China were admitted), only 43% had fever, only 67% had a cough.  (How did they detect them so early?  Aggressive tracking & testing of contacts.)    As with ALL these studies (China and other countries), "The uncertainty of the exact dates (recall bias) might have inevitably affected our assessment.'

Compare this to the report about the pre-symptomatic spread.  The incubation period minus the serial interval (interval between successive cases) was 2.5 days and 4.8 days, which, as I understand it, would be about how quickly someone becomes infectious after exposure.

[M T]

One thing to keep in mind is that the probability of infection is related to the number of viable viral particles that are transmitted to an infectable surface (I assume a mucous membrane) and how direct the transmission is.

A number of misunderstandings there, based on the medical articles I'm reading. 

Main mode of transmission is by inhaling droplets from the infected person's breathing.   Coughing/Sneezing is not needed (as aKidDoc said earlier).
In 2003's SARS-COV-1 days, at least one study in healthcare situations indicated failure to wear a mask resulted in more illnesses than failure to wear gloves.

There is no need for a very sick person to do the breathing.  The seemingly healthy guy at your door dropping off groceries can give you a dose.  The virus shedding is maximal early in the disease.   The evidence has been that a person is infections between 2 to 3 days before the first symptom.

There are cases where person A infects person B, and person B gets the symptoms well before person A.
The serial interval (time that B shows symptoms relative to A) is in the 4-5 day range, which is shorter than the incubation period.

---- details

In this Mar 13 preprint on serial interval (time between a series of infections),

Notably, 59 of the 468 reports indicate that the infectee developed symptoms earlier than the infector. Thus, pre-symptomatic transmission may be occurring, i.e., infected persons may be infectious before their symptoms appear.

the serial interval was as low as -11 days to as high as 20 days, with a mean of 4 days and a standard deviation of 4.75 days.

Wow!  I had seen indications of very short serial intervals (the Mar. 6 report that lead this particular thread had serial interval as low as 2+ days (ie, 2 days after infection of person A, person A infected person B))   Apparently, the variability in onset of symptoms (incubation) is large enough to allow B's symptoms to show up before A's symptoms.  


By the way, links to a number of papers can be found here.

China's 1099 patient study, the 291 for which documented exposure existed, had a mean incubation 4 days, with 25 percentile at 2 days or less, 75 percentile at 7 days.  At hospital admission (all positive tests in China were admitted), only 43% had fever, only 67% had a cough.  (How did they detect them so early?  Aggressive tracking & testing of contacts.)    As with ALL these studies (China and other countries), "The uncertainty of the exact dates (recall bias) might have inevitably affected our assessment.'

Compare this to the report about the pre-symptomatic spread.  The incubation period minus the serial interval (interval between successive cases) was 2.5 days and 4.8 days, which, as I understand it, would be about how quickly someone becomes infectious after exposure.