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Comment by Ankaios | Hacker News Reader

At least anecdotally, the mRNA vaccines do appear to provide some protection against infection. I know three people who are mRNA-vaccinated who were recently extensively exposed to omicron. (They spent multiple hours in a single room or car with someone with symptoms who was later confirmed infected.) The exposed individuals are PCR tested at least twice a week and almost certainly never previously had covid. One of the three later tested positive, but the other two never tested positive.

Maybe by sheer luck two out of three could have remained uninfected by omicron without being vaccinated, but I'd be pretty surprised.

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> I know multiple people who are PCR tested at least twice a week, who have never had covid, who had extensive exposure to omicron (multiple hours in a single room or car) who never tested positive afterward.

Do PCR tests indicate if you've ever been infected? I thought they only indicate if you're currently infected. In fact I'm curious if there is any test that can tell whether you've been infected by any variant of SARS-CoV-2 more than a few months ago; is there?

Your understanding is correct. PCR tests can only tell if you are currently infected. (Or at least have been so recently infected that enough remnant mRNA is still lingering in your body to be detectable.)

Antibody tests can determine whether someone was infected previously, and I have heard that there are some that look for antibodies that are associated with infection but not vaccination. I don't know anything more about them than that, but maybe that could provide a thread to help start searching if you are interested in finding out more about them.

Yeah I've looked that up before, and from what I recall reading, the antibody tests only work for "recent" infections in the last few months, not something that may have happened (say) a year or more ago. I don't remember reading that any test works for non-recent infections.

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.

Actually I just Googled again, and they simply say the precise duration is unknown [1]:

> IgG antibodies, including IgG against the S and N proteins, persist for at least several months in most persons, but the precise duration of time that antibodies persist after infection is unknown.

[1] https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/anti...

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