[KennyBlanken]

Nose swabs reveal whether you're shedding the particles and thus infectious. It doesn't matter if you're infected if you're not shedding the virus.

They are also exceptionally reliable. The home test kit I used had a sub-1% false negative rate.

[timr]

> Nose swabs reveal whether you're shedding the particles and thus infectious. It doesn't matter if you're infected if you're not shedding the virus.

No, they really don't. Swabbing for RNA picks up gene fragments that may or may not be from infectious virus -- it's why we see positive tests for months after infection in some people.

Swabbing for viral protein is debatably more likely to detect the thing of interest (the virus itself, in some semblance of functioning order), but these tests also have a high false-negative rate (around 10% for the better tests I've seen; I have never heard of a test with a sub-percent FN rate, as you claim). You can be shedding live virus and these tests won't pick it up, either because you're not shedding enough, or because the antibodies in the test don't bind to the protein in your sample for whatever reason.

Either way, you're measuring a proxy for what you really care about. A true test of infection involves taking a sample and incubating in cell culture. Nobody does this, except to validate the original tests and provide clear positive and negative samples. It's slow and orders of magnitude more expensive than even PCR testing. But this is the direct test for infectious virus. Everything else is an approximation.

(Let me be clear, though: I wholeheartedly support the use of antigen tests -- even ones with low sensitivity -- over the insanity we're doing now in the US. It's just bad to misrepresent what they're actually doing.)

[refurb]

Agree with all of this. PCR literally involves amplifying segments of genetic material so it can be detected. All you need is a segment of genetic material, not the whole virus.

However, I’m not sure the value in antigen testing? Sure, when you’re traveling or have to into a higher risk situation.

But Singapore decided to freely give out antigen tests and what happened was people who tested positive showed up at the ER. And the antigen tests weren’t reliable, so PCR had to confirm. And they have a high vaccination rate so after all that testing the answer was “go home and if you get really sick, come back”.

It finally dawned on them that could just be the message anyways - if you don’t feel bad, don’t worry. If you do, you can test but don’t seek medical care unless you have severe symptoms.

[timr]

The value of cheap, ubiquitous antigen testing is that you can be pretty sure that you don't have the virus, which allows scared people to have some sense of control. Even though these tests have a high false-positive rate, it's pretty unlikely that you'll test negative on multiple independent tests, so the cheap and ubiquitous part is important. Scared people can fixate their fear on a metric that actually correlates with transmission. Negative test? No need to freak out about going to the store.

That said, your point is well-taken that people can be idiots about testing positive. We do need to get over this fear and accept that the virus is endemic, and that vaccines work to prevent serious illness. We're now talking about miniscule risks that we would have rightfully shrugged off in any previous year, but folks have been terrorized, and they're desperately looking for control. Any tool that can calm that fear is a good tool.

[ssl232]

> Nose swabs reveal whether you're shedding the particles and thus infectious.

On the "shedding" point, not necessarily. The virus can be present in but contained by the immune response from the mucosae of the upper respiratory tract in such a way that it is unable to spread into the lungs and cause COVID-19, yet not shed in large enough quantities to infect others. Given time, a healthy immune system will deal with the virus in the nose and throat, often without the host even noticing. Such a situation would set off a PCR or rapid test but not present a meaningful COVID-19 infection risk to the others. (In fact, one hypothesis for why positive cases rise soon after vaccination and booster campaigns start is because of the well understood phenomenon of reduced immune response for a short time after vaccination, giving such virus already present in the upper respiratory tract at time of vaccination the edge it needs to get into the lungs.)

And the cycle thresholds on PCR tests are often set nonsensically high making them sensitive to quantities of virus and viral debris far lower than the quantity required to meaningfully infect either the host or someone else via shedding. They can also trigger positive on not just virus but viral debris for months after recovery from COVID-19 infection. (A test can be too sensitive, especially when used as the only evidence to force someone and their contacts to isolate and in some cases not earn an income for weeks.)

> It doesn't matter if you're infected if you're not shedding the virus.

I agree, but I'm not sure if the Belgian authorities, who seem to use PCR positives as a COVID-19 diagnosis, and PCR negatives as a guarantee of safety from infection risk to others, would. The article does what most articles these days do, conflating presence of SARS-CoV-2 debris on a swab with COVID-19 disease diagnosis. It incorrectly claims 2/3rds of the 25 staff have COVID-19, when given that none seem to have symptoms of the disease it's likely a case of oversensitive tests. Let's not also forget that these tests are mostly (at least all the ones I've seen) called COVID-19 tests.

[chana_masala]

> Given time, a healthy immune system will deal with the virus in the nose and throat, often without the host even noticing.

Ah ok, so that might explain why there's a significant number of people who say that they had covid without difficulty, at least of they didn't test false positive.

Thanks for explaining the nuance - I've heard a lot of this before but it's refreshing how succinctly you captured it.