[Coronavirus] Is it a good idea to meet people indoors if everyone's rapid antigen test came back negative?

post by Jonas Vollmer · 2021-03-24T15:45:58.043Z · EA · GW · 10 comments

This is a question post.

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:

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:

Answers

answer by Tsunayoshi · 2021-03-25T02:23:41.839Z · EA(p) · GW(p)

[Epistemic status: This is mostly hobbyist research that I did to evaluate which tests to buy for myself]

The numbers listed by the manufacturers are not very useful, sadly. These are generally provided without a standard protocol or independent evaluation, and can be assumed to be a best case scenario in a sample of symptomatic individuals. On the other hand, as you note, the sensitivity of antigen tests increases when infectiousness is high.

 I am absolutely out of depth trying to balance these two factors, but luckily an empirical study from the UK  estimates based on contact tracing data that "The most and least sensitive LFDs [a type of rapid antigen tests used in the UK] would detect 90.5% (95%CI 90.1-90.8%) and 83.7% (83.2-84.1%) of cases with PCR-positive contacts respectively." So, if a person tests negative but is still Covid-19 positive, you can assume the likelihood of infection to be 10-20% of an average Covid-19 contact.  

With regards to self vs. professional testing, there does not seem to be a very clear picture yet, but this German study suggests basically equivalent sensitivity.    

You should also make sure to buy tests that were independently evaluated, you can find lists of such tests here or here. The listed numbers are hard to compare between different studies and tests, however, but the one you mentioned seems to have good results compared to other tests

I am honestly not sure how long the test results are valid, but 2 hours seems safe. I cannot comment on the other numbers provided by microCovid. 

comment by JP Addison (jpaddison) · 2021-03-26T11:24:51.791Z · EA(p) · GW(p)

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.

comment by Jonas Vollmer · 2021-03-25T08:38:21.023Z · EA(p) · GW(p)

Thanks, this is helpful! Feel free to PM me your payment details so I can send you the $100 reward mentioned in the post.

answer by atlas · 2021-03-26T09:23:28.436Z · EA(p) · GW(p)

[Another hobbyist here]

I agree with Tsunayoshi's answer.

Another thing to keep in mind that even the best studies on rapid antigen tests usually compare against PCR tests; that is, if they agree with PCR tests in all cases, the sensitivity is reported as 100%. However, the sensitivity of PCR tests is (as far as I can tell) not 100%, and can vary a lot based on factors such as how the sample is collected and transported.

Here's an article on the issue. Key quote:

Whether a SARS-CoV-2 test detects clinical disease depends on biologic factors, pre-analytic factors, and analytic performance. Someone with a large amount of virus in their nose/throat will have a positive test with a nose/throat swab. However, someone with little to no virus in their nose or throat may have a negative test even if they have virus somewhere else (like the lungs). [...] If no virus is present at the site of collection, the collection fails to get virus in the sample, or the sample is severely degraded from storage or transport (for example baking in the sun on a car dash) then the test will be negative no matter how sensitive the test is.

Then there's studies like Kucirka et al, which is summarized in a later paper via this graph of false negative rates in PCR tests:

The study concludes

If clinical suspicion is high, infection should not be ruled out on the basis of RT-PCR alone, and the clinical and epidemiologic situation should be carefully considered.

I don't know how trustworthy the Kucirka et al study is, since the false negative rates reported are a lot worse than any I've seen elsewhere. But I think the upshot is that even "gold-standard" PCR testing is messy, and we shouldn't trust studies that estimate antigen-test sensitivity by comparison to PCR (or at least adjust for low PCR sensitivity).

A different conclusion that I think is reasonable is that RT-PCR tests are a good baseline given competent administration and possibly re-testing. I don't know enough about the mechanics of testing to evaluate whether a given study does well on this or not. 

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comment by Lukas_Gloor · 2021-04-23T10:21:10.752Z · EA(p) · GW(p)

This tweet (in German) seems relevant.

And here's a related anecdote: This story might just be a fluke, but it does suggest that it can happen that people test negative repeatedly shortly before superspreading.

comment by Max_Daniel · 2021-03-24T15:55:16.782Z · EA(p) · GW(p)
  • Someone suggested that tests tend to have higher sensitivity for highly infectious people, so the officially reported prevalence might actually be too low for our purposes.

In the 2nd part of the sentence, did you mean to say "sensitivity" rather than "prevalence"?

Replies from: Jonas Vollmer
comment by Jonas Vollmer · 2021-03-24T16:14:27.282Z · EA(p) · GW(p)

Oops, yes, edited.

comment by Manuel_Allgaier · 2021-04-13T11:15:07.254Z · EA(p) · GW(p)
  • So meeting someone who tested negative indoors without a mask implies 18%*.02% = 36 microCOVIDs, which is very low.

I'd also be curious to know better estimates to this - have you updated your estimate after these two answers? Or did anyone else update their estimates?

(if you have time to reply, no worries if not)

Replies from: Jonas Vollmer
comment by Jonas Vollmer · 2021-04-16T17:25:20.348Z · EA(p) · GW(p)

Sensitivity is relative to PCR tests, and tends to be reported quite incorrectly. So unlike I suggested in the OP, I think the adjustment should probably be ~4x and not ~12x.

comment by Maxime_Riche · 2021-03-25T09:07:20.922Z · EA(p) · GW(p)

Could the odds be dominated by the symptom starting during the meeting (or shortly before it)? For that to be clearly false, the antigen test should be effective before the person is contagious. If the test is taken 6h before the meeting then the test should be reliably effective 6h before being contagious.