0.5% IFR is still very high, I wonder what the antibody detection threshold is - i.e. is it possible that a lot more people have been exposed but don't have det

0.5% IFR is still very high, I wonder what the antibody detection threshold is - i.e. is it possible that a lot more people have been exposed but don't have detectable antibody levels ? Would it qualify as infection in the sense that they are not susceptible for covid19 in the near term ?