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Re: COVID [was: Re: DebConf 25 Daily announcements - 2025.07.15 - Daytrip information && DebConf Day 2]



On Friday, July 18, 2025 4:49:24 p.m. Central European Summer Time Julien Plissonneau Duquène wrote:
> Le 2025-07-18 08:29, Alex Lieflander a écrit :
> 
> I would like to remind here (and this is not make it whataboutist 
> fallacy) that statistically it is almost certain that several persons 
> infected with COVID are currently walking around in the venue and 
> spreading the disease:
> - people that are infected and won't have any symptom (according to 
> doctors, with the current variants this happens fairly often)
> - people that are infected and don't yet have symptoms
> - people that have symptoms and ignore them, because they are very mild 
> and look just like a common cold (as with my own case) or something else 
> (as with the local strain that is heavier on GI symptoms)
> - people that unquestionably have some COVID symptoms and choose to 
> ignore them, because they want to avoid the consequences (from 
> additional distancing to self-isolation)
> 
> There is nothing that can be done about this, short of testing every 
> person every single day, and even then some cases would still escape the 
> screening (the rate of false negatives with these antigenic self-tests 
> is low but still significant).
> 
> If you think that having masked and careful COVID-infected persons 
> around makes a significant difference in terms of risks, I suspect that 
> you actually severely underestimate your current exposure to 
> contamination with the rest of the crowd around.

Here's an example calculation of the risk with some **rough** approximations. Let's assume that:

- There are 400 Debconf attendees
- 5% of Debconf attendees currently have COVID
- 20% of people with COVID are either asymptomatic or ignore their symptoms
- COVID tests give a false negative 10% of the time
- 100% of people who know they have COVID wear a mask at all times
- 0% of unknowingly COVID-positive people wear masks
- The probability of a particular person being infected by an unmasked person with COVID is 5%
- The probability of a particular person being infected by a COVID positive person wearing a mask is 1%

Of the 400 attendees, 20 would have COVID and 16 of those people would have symptoms and get tested. 2 people would get a false negative, so there would be 6 unknowingly COVID-positive people (4 asymptomatic + 2 false negatives) and 14 people who know they have COVID. Since the probability of a positive unmasked person infecting a particular person is 5%, the probability of an unmasked person NOT infecting that person is 95%. The probability that NEITHER the first person, NOR the second person, NOR the third, NOR the fourth (etc) infect that person is 0.95^6=0.74 or 74%.

If the knowingly positive people self-isolate, their chance of infecting people is basically 0%. If they continue to attend the conference while wearing masks, the probability that none of them infect a particular person is 0.99^14=0.87 or 87%. The probability that neither group infects a particular person is 0.74*0.87=0.64 or 64%. That means that if everyone who knows they have COVID self-isolates, each healthy attendee would have a 26% probability of being infected. If everyone who knows they have COVID continues to attend the conference, each healthy attendee has a 46% probability of being infected.

TL;DR: Even assuming some asymptomatic people and some false negative tests, if people who know they have COVID just wear a mask instead of self-isolating, each healthy attendee would be 76% more likely **relative to the other option** to be infected. I'm not a doctor or an epidemiologist, but I do know a bit about statistics.

Alex



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