Yeah, there's a limited window over which antibody testing is useful. I hadn't heard that it was only a few months, but that wouldn't be shocking, especially if you want a low rate of false negatives. Hopefully someone reading this knows more than me about approaches to testing that might work over longer time scales and not be confused by vaccination.
Serological tests are useful and needed right now, not at some future stage.
It takes about a month before an antibody test is accurate in someone who has had a virus, so using them now would only detect cases where the person had the virus in early March. That limits the value of serological testing considerably.
So because this tests for antibodies, are you saying someone could test positive, but it's because they got it like a month ago? (e.g. this is more similar to those tests you have to take before you start at a hospital; so they can see what booster vaccines you need?)
Yep, the fundamental problem is that the antibody test only triggers positive well after you would already be infectious probably for days, and most likely symptomatic already at least. So it can't curtail the outbreak, though as others are pointing out it can be very useful to clear people after they are infected.
A test months later would be negative (if it was a swab) they are only accurate in the first week or so.
The antibody tests are fraught with issues, and if you fight the virus with your T cells rather than B cells, the antibody test won't pick that up and you'll show as negative.
> It takes a long time for the body to produce the antibodies after infection. This test is only 90% sensitive after 12 weeks
That's not much of a problem except for people worried about infection after a single known exposure (e.g. needlestick in a medical setting).
About a third of patients spontaneously clear the acute infection within six months. On top of that, some countries (such as Germany) refuse to treat acute infections with antivirals - so patients have to wait 6 months for the infection to develop into a chronic infection before they can take the medicine which cures it.
Issue with this (and other antibody tests) is both that levels of antibody vary wildly from person to person and across time.
The same person can show positive for a swab test, and then negative for antibody tests at months one and 2 and only start to show at month 3.
Equally some cases will never be revealed by an antibody test as the patient fought the virus solely with their T-cells - not antibodies.
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This obviously doesn't mean that a cheap and reliable test is not of use - just that it will no doubt reveal more issues and edge cases with our understanding of the virus.
A bigger issue than the specificity of the antibody tests is that they're generally not positive until you're nearly recovered. This confines their use to specific purposes (assessing whether someone may be immune, estimating true infection fatality rates, etc).
What they might have meant is that many countries accept a positive PCR from >= 4 weeks ago as proof of prior infection (and therefore, presumed immunity), but not an antibody test. I'm not familiar with the error rates of the latter, but for the purposes of travel a positive antibody test is rather useless.
Where I live, you need to meet one of three conditions for things like indoor dining:
- proof of a full course of vaccination (2 weeks after the final shot)
- proof of prior infection - antibody test not accepted, only positive PCR between 1 and 6 months ago
- proof of an antigen test (rapid test) that's at most 24h old
For what? "Google for answers" isn't very helpful. It sounds like they're saying serological tests are valuable, but not viable for immediately testing a large population.
The antigen test also seems to be ineffective at detecting igg antibodies months after infection due to their half life and very few non clinically researched focused tests for t or b cells seem to have been carried out anywhere
How would they know to use the antibodies if they didn't test people first? The [2] article you linked refer to vaccines not tests. I think you're missing something in your explanation here.
What's the point of antibody testing these days, since everyone who's been vaccinated will test positive, and basically everyone's been forced to get the vaccine by now?
The other issue is that we don't know what concentration of antibodies is required for immunity. We know these things for other diseases, like influenza, chickenpox, etc. We have no point of reference, so we can only test for the presence of antibodies.
I think antibody tests are still likely to be useful, especially in collecting data on the pervasiveness of the disease and getting information to find out what threshold immunity occurs at. The messaging around them is rife for misuse, though.
Everyone is looking for a silver bullet, but all we can do right now is the long, slog of science to get to a place where it's truly safe to try to return to a semblance of normalcy.
The half-life of antibody levels has been estimated at 73 days. The antibody tests are quite sensitive, time has not been enough of a factor yet to produce false negatives. In a year or two it might be.
The only time concern is that you need to wait 2-3 weeks until after you recover from COVID to ensure you test positive.
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