I can’t read this NY Times article because of the paywall, but you might be interested, as it is on this topic:
https://www.nytimes.com/2021/06/20/health/covid-vaccine-antibody-test.html
Mr. Lat turned to Twitter for an explanation. Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York, responded, asking Mr. Lat which test had been used. “That’s when I looked at the fine print on the test,” Mr. Lat said. He realized it was a test for antibodies to the N protein, not to the spike.
My fine print says:
Clinical Significance
SARS-CoV-2 Antibody (IgG), Spike, Semi-Quantitative - This test is intended as an aid in identifying individuals with an adaptive immune response to SARS-CoV-2 (COVID-19), indicating recent or prior infection. The results of this semi-quantitative test should not be interpreted as an indication of degree of immunity or protection from reinfection.
Individuals who have been vaccinated with a SARS-CoV-2 spike or receptor-binding domain vaccine may be positive with this test; however, the clinical significance of a positive antibody result for individuals who have received a COVID-19 vaccine is unknown, in part because the test's performance characteristics have not been established for that population. It is not known how long antibodies persist following infection and if the presence of antibodies confers protective immunity. This assay should not be used to diagnose acute SARS-CoV-2 infection.