Question: After having had both doses of the Pfizer vaccine, and having given it a couple of weeks to register in my system, would it be a good idea to get an antibody test for proof?

Answer: CDC does *not* recommend antibody testing to assess for immunity after vaccination. A recent Washington Post article provides additional information supporting CDC’s guidance.  In short, it seems highly likely that you’ll get a false negative result.  Given the strength of the Pfizer (and Moderna) vaccines in clinical trials, you can consider yourself immunized and safe from moderate and severe disease 3 weeks after the second shot.  Because we still don’t know whether immunized people can carry the virus and transmit it to others, please keep up with good public health practices even after you’re vaccinated– masking, social distancing, hand washing, avoiding crowded places, close contact settings, and closed spaces.  If you’d like to read more, I’ve included a bit more detail on the antibody testing issue in the below paragraph. 

Why not get an antibody test? After vaccination with Pfizer or Moderna, the antibodies formed will be to only one part of the virus– the spike protein.  When it comes to COVID antibody tests, there are generally two types– those that detect antibodies to the nucleocapsid protein and those that detect antibodies to the spike protein.  Tests that detect nucleocapsid protein antibodies will not pick up any markers of vaccine-derived immunity.  Meanwhile, scientists aren’t sure how well spike protein antibody tests will fare at detecting markers of vaccine-derived immunity.  Plus, a positive antibody test from such a test cannot differentiate between previous infection and vaccine-derived immunity.  Antibody tests are further limited because they only measure one component of immunity (antibodies) but not others (e.g. T-cells).  All that to say, antibody tests are likely to give false negative results; even when they give true positive results, you can’t be sure that it’s due to vaccine-derived immunity; and even if they give true negative results, you can’t be sure that it means you aren’t protected against COVID (hello, T-cells).