2/ Commercially available tests (what you'd get through your doc's office / urgent care clinic) measure binding, not neutralizing antibodies.
While there is a correlation, it's an imperfect one.
We don't know what level of neutralizing antibodies are necessary for protection.
3/ Some tests only pick up antibodies after infection (e.g. antibodies to nucleocapside protein).
Others pick up antibodies after infection or vaccination (e.g. antibodies to spike protein).
4/ You could have a negative COVID antibody test after vaccination because you were tested for nucleocapsid protein.
5/ Even if we knew what level of Spike protein neutralizing antibodies were protective, laboratory tests haven't been standardized to allow for cross-comparison.
These tests need to be standardized and calibrated (as has been done for measles, etc).
6/ Antibodies also aren't the only arm of the immune system that protect against COVID. Your memory B-cells stick around for much longer, even after antibody levels drop:
1/ There's good evidence that to protect against severe disease, hospitalization, & death, the following would benefit from an additional dose of COVID vaccine:
- immunocompromised persons
- people 60-65+ (? 40+)
- residents of long-term care facilities
3/ What should our goal be with the vaccines?
- Prevention of severe disease, hospitalization, & death
- Prevention of symptomatic infection (which means trying to prevent ALL infections since we can't always predict who'll be symptomatic)