Antibody Tests after Vaccination

The CDC recently updated guidelines for interpretation of SARS-CoV-2 antibody tests after infection and vaccination.  There is currently no recommendation to measure antibodies after vaccination to determine vaccine effectiveness.  Despite this, I know vaccinated individuals who measure antibodies anyway, and they are surprised when antibody tests come back negative.

Okay, full disclosure.  It was me.  And my wife.  We took the Janssen vaccine in mid-March, and I checked our SARS CoV-2 IgG antibodies last week.  They’re negative.  For both of us.  What’s going on here?  Isn’t the point of vaccination to stimulate the production of antibodies?

The answer is a qualified yes.  The qualification comes in two parts: the “scientific explanation” and my opinion.  

First the “scientific explanation”, simplified.  An individual may produce three types of antibodies against SARS-CoV-2: N, S or RBD. Infection stimulates the production of all three types of antibodies, but vaccine stimulates the production of S antibodies only.  Therefore, vaccinated individuals who have never been infected will be S-antibody positive, but N and RBD antibody negative.  If the antibody test is designed to detect N or RBD antibodies, but not S antibodies, then the result will be negative in those individuals.  On the other hand, detection of N or RBD antibodies in a vaccinated individual means that the individual has been exposed to the virus, either before or after vaccination.  The trouble is that the antibody tests available today by emergency authorizations do not specify which type of antibody is measured.   We do not know which antibody we are measuring unless we know details about the test, details that are usually not listed on the report from the lab.  

This explanation has precedent.  We know, for example, that Hepatitis B infection stimulates the production of two different types of antibodies: core antibody and surface antibody.  Hepatitis B vaccine only stimulates production of surface antibody; it does not stimulate production of core antibody.  Therefore, someone who is positive for surface antibody only has been vaccinated but has not been infected.  On the other hand, someone who is positive for both surface and core antibodies has been infected by Hepatitis B in the past.

All well and good for hepatitis, but what about SARS-CoV-2?  In the case of SARS-CoV-2, the “scientific explanation” is a hypothesis, meaning it is an educated guess.  Before we can say that we know something, we must compare our hypothesis to real-world observations.  There has not been enough time to observe the response that our bodies really have to vaccine.  It doesn’t make the hypothesis wrong, but it doesn’t make it right either.

My opinion?  The “scientific explanation” is still an opinion.