Antibody tests to assess immunity after Covid-19 vaccinations

Have received many Qs lately on SARS-CoV-2 antibody tests to assess immunity after Covid-19 vaccination. While these are available commercially across Canada and the US, we are far from using it ubiquitously and not currently recommended , because there are quite a few issues that haven’t yet been worked out yet, such as:

1- Most private antibody serological tests test for binding antibodies. These detect both SARS-CoV-2 spike (S) protein and the nucleocapsid ( N ) protein. Importantly, infections result in both N and S protein whereas vaccines only create S protein. So when you get your result, it is difficult to differentiate whether these antibodies are from previous infections or vaccinations. Neutralizing antibody tests are much more difficult and expensive to do because they require either a live virus or a pseudovirus.

2- These also only test only for binding antibodies and not T-cells. Remember, even if antibodies are low, it doesn’t necessarily mean you’re not protected. T-cells, in fact, play a huge role in long-term immunity.

3- We don’t know how valid the results are, ie, what the sensitivity or specificity and ultimately, how robust the results from these tests are.

4- Most importantly, we also don’t really know what level of antibodies constitutes ‘protection’ – ie does a positive test mean you are protected?

5- We don’t know if the results would impact clinical decisions at this point. ie, should someone get a third booster based on the results?