[M T]

Just because we can't contact trace doesn't mean it can't be done in other countries.  Here are the results of contact tracing in China:

https://www.medrxiv.org/content/10.1101/...20053058v1

Among the identified outbreaks, 53.8% involved three cases, 26.4% involved four cases, and only 1.6% involved ten or more cases. Home outbreaks were the dominant category (254 of 318 outbreaks; 79.9%), followed by transport (108; 34.0%; note that many outbreaks involved more than one venue category). Most home outbreaks involved three to five cases. We identified only a single outbreak in an outdoor environment, which involved two cases.

First, they only looked at outbreaks with 3 or more cases.  That assures that anyone individually affected was excluded; their selection method seems to require a place.  Pedestrians walking to work would probably not been recognized as an outbreak.

They covered 1245 positive cases (all outside Hubei) January 4 to Feb. 11.  It isn't clear whether the cases were confirmed by that date or the exposures happened before that date, or the case tracking reports were filed by that date.  As of 24:00 Feb 11, mainland China excluding Hubei province reported 11,286 confirmed cases with more than 1,000 a day newly confirmed.   So they looked at a fraction of the cases.   The quality of the data they used is unknown (to us, at least).  For instance, if 5 people that got sick rode a bus on a particular day, did they identify the infector and whether the exposure happened in the bus or while waiting for the bus, or just assumed it was on the bus?

It is unclear to me how much outdoor activity happens in China during January to February.  I know zilch about the weather in China, so I'll just use Beijing Airport's weather.  Four of the about six weeks had an average temperature range roughly 25-41F.  The other two weeks were colder (average high around freezing).  It was mostly dry. 

I don't recall that the study reported what percentage of time was spent indoors versus outdoors. If the average person's time outdoors was 10 minutes a day, then it would be very surprising if any outdoor infections were found.

and Japan:
https://www.medrxiv.org/content/10.1101/...20029272v2

The odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment (95% confidence interval [CI]: 6.0, 57.9).

If you spend 73 minutes a day outdoors, the odds that any instant happens when you are indoors is 18.7 times greater compared to it being outdoors.  

If you want to say that a person is exposed to infectors for (say) 12 hours a day, then make that 36 minutes outdoors.

This study was apparently made public on Feb. 28. The first outbreak in Japan was Jan. 16.  Again, just using a single location for a whole country is woefully inadequate, but Narita Airport average temperature was in the low to upper 40s (F) each week during that time.  (Note that skiers spend a lot of time outdoors at much colder temperatures.)

Neither of these studies evaluated the relative risk per unit time spent outdoors versus indoors.  Neither reported (even from survey of the general population) a ratio of time spent indoors versus outdoors.   A better study would evaluate the time exposed to others, and how much of the time was indoors or outdoors.   Even better would be if it studied people before hand, and then followed up later to see if they got sick.

Neither study shows me that outdoors is any safer than indoors.  Their results could have been entirely due to the population spending more time indoors than outdoors. So I will accede to "Most transmission is indoors" when most time is indoors.  

I don't get impressed by 3 digits of precision.

[M T]

Just because we can't contact trace doesn't mean it can't be done in other countries.  Here are the results of contact tracing in China:

https://www.medrxiv.org/content/10.1101/...20053058v1

Among the identified outbreaks, 53.8% involved three cases, 26.4% involved four cases, and only 1.6% involved ten or more cases. Home outbreaks were the dominant category (254 of 318 outbreaks; 79.9%), followed by transport (108; 34.0%; note that many outbreaks involved more than one venue category). Most home outbreaks involved three to five cases. We identified only a single outbreak in an outdoor environment, which involved two cases.

First, they only looked at outbreaks with 3 or more cases.  That assures that anyone individually affected was excluded; their selection method seems to require a place.  Pedestrians walking to work would probably not been recognized as an outbreak.

They covered 1245 positive cases (all outside Hubei) January 4 to Feb. 11.  It isn't clear whether the cases were confirmed by that date or the exposures happened before that date, or the case tracking reports were filed by that date.  As of 24:00 Feb 11, mainland China excluding Hubei province reported 11,286 confirmed cases with more than 1,000 a day newly confirmed.   So they looked at a fraction of the cases.   The quality of the data they used is unknown (to us, at least).  For instance, if 5 people that got sick rode a bus on a particular day, did they identify the infector and whether the exposure happened in the bus or while waiting for the bus, or just assumed it was on the bus?

It is unclear to me how much outdoor activity happens in China during January to February.  I know zilch about the weather in China, so I'll just use Beijing Airport's weather.  Four of the about six weeks had an average temperature range roughly 25-41F.  The other two weeks were colder (average high around freezing).  It was mostly dry. 

I don't recall that the study reported what percentage of time was spent indoors versus outdoors. If the average person's time outdoors was 10 minutes a day, then it would be very surprising if any outdoor infections were found.

and Japan:
https://www.medrxiv.org/content/10.1101/...20029272v2

The odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment (95% confidence interval [CI]: 6.0, 57.9).

If you spend 73 minutes a day outdoors, the odds that any instant happens when you are indoors is 18.7 times greater compared to it being outdoors.  

If you want to say that a person is exposed to infectors for (say) 12 hours a day, then make that 36 minutes outdoors.

This study was apparently made public on Feb. 28. The first outbreak in Japan was Jan. 16.  Again, just using a single location for a whole country is woefully inadequate, but Narita Airport average temperature was in the low to upper 40s (F) each week during that time.  (Note that skiers spend a lot of time outdoors at much colder temperatures.)

Neither of these studies evaluated the relative risk per unit time spent outdoors versus indoors.  Neither reported (even from survey of the general population) a ratio of time spent indoors versus outdoors.   A better study would evaluate the time exposed to others, and how much of the time was indoors or outdoors.   Even better would be if it studied people before hand, and then followed up later to see if they got sick.

Neither study shows me that outdoors is any safer than indoors.  Their results could have been entirely due to the population spending more time indoors than outdoors. So I will accede to "Most transmission is indoors" when most time is indoors.  

I don't get impressed by 3 digits of precision.