reading the FDA news release it seems that the basis for a booster is waning neutralizing antibody levels
https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/antibody-serology-testing-covid-19-information-patients-and-consumers?msclkid=5be5d442b04911ecb4cc5aec06c01bffAntibody Tests: Not for Use to Check Immunity
Q: Will a positive result on a SARS-CoV-2 antibody test mean I have immunity and I will not get COVID-19?
A: No. At this time, SARS-CoV-2 antibody tests do not tell you if you have immunity that will prevent you from getting COVID-19. A positive SARS-CoV-2 antibody test does not necessarily mean you are immune or have immunity that will prevent COVID-19. More research is needed to understand what SARS-CoV-2 antibody test results can tell us. And, SARS-CoV-2 antibodies detected in your blood reflect only one part of your immune system, which also includes T-cells and other components that are part of your body's immune response.
Q: Should I get a SARS-CoV-2 antibody test to decide if I need a COVID-19 vaccine or after I am vaccinated to see if my vaccine worked?
A: No.
At this time, antibody test results should not be used to decide if you need a COVID-19 vaccine or a vaccine booster, or to determine whether your vaccine worked. There is not a clear connection between SARS-CoV-2 antibody test results, the need for a COVID-19 vaccine or booster, or whether a vaccine worked in a person. Also, some SARS-CoV-2 antibody tests may not detect the kind of antibodies created following vaccination. SARS-CoV-2 antibodies detected in your blood reflect only one part of your immune system, which also includes T-cells and other components that are part of your body's immune response.
More research is needed to understand the role of SARS-CoV-2 antibody testing in evaluating a person's immunity or protection against COVID-19 and understanding if antibody tests will be helpful for deciding if a person should receive a COVID-19 vaccine. If you have questions about whether a SARS-CoV-2 antibody test is right for you, talk with your health care provider or your state or local health department.
Q. Can a SARS-CoV-2 antibody test tell me if I could infect other people?
A: No. Antibody tests do not tell you whether or not you can infect other people with SARS-CoV-2. Current information indicates people infected with SARS-CoV-2 can still transmit the SARS-CoV-2 virus and infect other people, even if they are COVID-19 vaccinated or have detectable SARS-CoV-2 antibodies from a previous infection.
Poll