Can antibody tests determine if the COVID-19 vaccine was effective?

The Centers for Disease Control and Prevention (CDC) discourages antibody testing for accessing immunity after getting the vaccine. The major reasons are: 1. Different serology tests detect antibodies to different parts of the virus. The vaccine causes immune response to spike protein, therefore only antibody test against spike protein can detect the immune response to the vaccine. 2. The antibody level is too low to be detected at the time of testing. Individual’s response to the vaccination is different. In general, it takes two to three weeks after vaccination for antibody to be detectable. It may take longer for some individuals to have detectable antibody after vaccination. Therefore, one time test can’t be used to assess the effectiveness of the vaccination.