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(05-10-2020, 05:02 PM)M T Wrote: On Friday, someone, unofficially (hence "someone"), indicated the infection rate (ever infected, not currently infectious) of tested staff at a Bay Area hospital was less than 1% (offered as a counterpoint to the Stanford antibody study).
I've been thinking about the prevalence of the infectiousness for someone you come across in Santa Clara County. Currently, every day, about 1 in 100,000 people in SCC test positive. Each case is infectious and not in quarantine for about 5 days (a few days before symptoms + a few days after symptoms before realizing you are really sick). So, about 1 in 20,000 people you might run into is infectious. If the 1 in 100K are all symptomatic, and if there is as many new asymptomatic cases as new symptomatic, and we assume the asymptomatic are infectious for 10 days, then 1 in 6,700 people are infectious. That's all back-of-the-envelope, rough calculations.
I ponder the same question when I go out, but I come up with a more pessimistic number if I use your approach. Assuming your # days infectious and ratio of symptomatic/asymptomatic are correct, your calculations assume 100% of symptomatic people eventually are tested and reflected in confirmed cases, but I think there are still many mildly symptomatic people who are never tested. Early on, who knows, maybe only 1 in 10 or 1 in 60 symptomatic people were getting tested because there were too few tests. SCC is still trying to set up more testing sites to reach more of the population, still trying to get more test supplies, and still hasn’t communicated clearly how everyone will get tested, how it will be paid for, where to go, etc. My guess would be still only 1 in around 5 symptomatic people in SCC are getting tested, that’s just a guess. I’d assume we are testing a much higher percentage of actual infections now because the % positive for tests used to be around 10% in SCC and now looks to be under 2%.
But if we’re only testing and confirming 1 in 5 symptomatic infections, maybe closer to 1 in 1,000 people are infectious, assuming the rest of your numbers are right. But I think that is a very scary way to look at things. It’ll hopefully convince people not to stand in a packed arena shoulder to shoulder with strangers for hours, but in everyday situations, even if 1,000 shoppers and joggers pass nearby every 10 days, it’s extremely unlikely that 1 puff of breath is going to get someone sick. Plus you’d hope not all the symptomatic people are the ones going out.
Here’s another way to crunch SCC’s numbers that comes up with a sunnier picture: if only 1 in 100,000 people in SCC get sick enough to get tested per day, you could go 200 days with the current case counts and you’d have only a 1 in 500 random chance of getting sick enough to end up getting tested (or, a 99.8% probability of not getting sick enough to be a confirmed case). And the daily case counts in SCC are dropping — hopefully the average for the next 200 days will be only 1/10 or 1/20 the current rate. So hopefully, in SCC maybe the odds will be 99.98+% you can avoid getting sick enough that you’d test positive during the next 200 days, assuming we can drive the case counts low and keep them low even during a partial reopening. But then, flu season will be here...