[1] https://www.stanforddaily.com/2020/04/04/stanford-researcher...
Antibodies are unique for each virus, but there can be "cross-reactivity" where an antibody for a different virus also binds to COVID-19.
Yes, it is possible that there are people out there who have antibodies that bind to COVID-19, who have never been infected with it, but rather have an antibody that cross-reacts with COVID-19. But it would be a very rare occurrence.
So having COVID-19 antibodies isn't a perfect was to tell if a person has been exposed, but it's a very good way to do it since antibodies is highly correlated with exposure.
It’s similar to the AAV (adeno-associated virus), a common virus.
There are multiple subtypes, but often an antibody to one will bind to other variants, making you immune.
> The term vaccine derives from the Latin word for cow, reflecting the origins of smallpox vaccination.
> We’re also concerned about the related coronaviruses that cause the common cold in humans. These viruses induce antibody responses that, at least temporarily “neutralize” the virus, but they don’t appear to last. Because the neutralizing antibodies wane over the course of 1 to 2 years, people can be reinfected with the exact same coronavirus 1 or 2 years later. The simplistic idea is that people are going to get infected with SARS-CoV-2 and then they’ll be resistant to infection for the rest of their lives, and that herd immunity will accumulate over time, etc. But if this is anything like the more traditional coronaviruses that cause common colds, you’ll get robust neutralizing activity at first, but it will wane over time. And so our studies are really focused on assessing that issue of persistence of immunity from reinfection.
[...]
> Q: What happens when SARS-CoV-2 infects a person who has antibodies to the other four coronaviruses that infect humans and cause the common cold?
> A: We were on a call today with the CDC about this. If we look at people who just went through a SARS-CoV-2 infection and have a burst of antibodies against the virus, they’ve also boosted their pre-existing antibodies against the classic cold coronavirus. And the earliest antibody responses that CDC researchers have seen in careful longitudinal studies to SARS-CoV-2 are actually those cross-reactive memory responses to the classic cold coronaviruses.
> Q: How might these cross-reactive antibody responses matter?
> A: The immune memory to previous infections may help control infection with those cold viruses and even ameliorate symptoms of SARS-CoV-2 infection. But it can cause problems with the accuracy of SARS-CoV-2 diagnostics, as people reinfected with common cold coronaviruses could score as false positive with some SARS-CoV-2 serological assays.
https://kidshealth.org/en/parents/test-immunoglobulins.html
Specialized versions are produced in response to individual disease. They are similar to each other but not the same.
https://premierbiotech.com/innovation/covid-19/
They're talking about it being a few days before they release the statistics from the testing.