Does anyone know how risky it is to meet someone indoors without mask if both take a rapid antigen Covid test and it comes back negative?
Rapid antigen tests are available in retail in some countries and tend to be pretty cheap (6€ per test, sometimes free). They seem easily worth the cost if it means one can hang out in person indoors.
My current thinking:
- The antigen test I looked at reports a sensitivity of 92% (95% CI: 83.63%-96.28%) and specificity of 99.26% (95% CI: 95.92%-99.87%)
- I take the point estimates at face value (don't really see a reason not to – but perhaps that's wrong?). The point estimates imply an odds ratio of 12 – if the prior (e.g. adjusted prevalence from microCOVID) is 0.4%, the posterior probability of that person having Covid is 0.03%.
- I do another arbitrary 1.5x adjustment because they don't have symptoms, so the posterior probability goes down to 0.02%.
- I assume (as per microCOVID) that the risk of an unmasked indoor hangout is 9% per hour, so 18% for 2h.
- So meeting someone who tested negative indoors without a mask implies 18%*.02% = 36 microCOVIDs, which is very low.
I would find it really useful to have an answer to this question, so I'm happy to reward good responses with $100 of personal money (up to $300 total).
Ways in which this could be wrong:
- Perhaps some rapid antigen Covid tests are much more reliable than others. How much attention should I pay to the type of test?
- People seem much more willing to take tests with a nasal swab that only goes 3cm deep rather than 10cm, as the latter tends to be painful for hours, whereas the former is not a big issue. But perhaps the more painful tests are better?
- Are there differences between self-administered tests and tests taken at a testing center? The former are obviously much easier to take.
- Maybe we should be worried about not being able to administer the test properly (e.g. if taking it outdoors, temperature might be too low?)
- Maybe there are reasons to think sensitivity/specificity are overestimates (like given that only some tests make it to market, perhaps there's some optimizer's curse issue going on?)
- Maybe the test tells you whether someone has Covid at the time of taking the test according to PCR, but they might become positive over the, say, 2h of meeting them, and maybe this issue is significant.
- Someone suggested that tests tend to have higher sensitivity for highly infectious people, so the officially reported sensitivity might actually be too low for our purposes.
PCR tests themselves aren't that sensitive, as mentioned by atlas. I've seen estimates (1, 2) of ~20% false negative. Counterbalancing that is that I assume people who have so little virus in their nose + throat to avoid a PCR test are at lower risk of spreading. But I would (very subjectively) record my microcovids as .3 x raw numbers.