Evolution of Anti-SARS-CoV-2 IgG Antibody and IgG Avidity Post Pfizer and Moderna mRNA Vaccinations

Messenger RNA (mRNA) based vaccines (Pfizer/BioNTech and Moderna) are highly effective at providing immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, there is uncertainty about the duration of immunity, evolution of IgG antibody levels and IgG avidity (an index of antibody-antigen binding strength), and differences in the immune responses between vaccines. Here we performed a prospective pilot study of 71 previously COVID-19 free subjects upon receiving both doses of either the Pfizer (n = 54) or Moderna (n = 17) mRNA vaccine. Anti-spike protein receptor binding domain (RBD) IgG antibodies were measured longitudinally using a qualitative finger stick MidaSpot rapid test at the point-of-care for initial screening and a quantitative dry blood spot-based pGOLD laboratory test over [~] four months post-vaccination. The average anti-RBD IgG antibody levels peaked at [~] two weeks after the second dose of the vaccine and declined thereafter, while antibody avidity increased, suggesting antibody maturation. Moderna vaccine recipients compared to Pfizer vaccine recipients exhibited higher side effect severity, higher peak anti-RBD IgG antibody levels, and higher avidity up to the 90 days period. Differences in antibody levels diminished at [~] 120 days post-vaccination, in line with the similar efficacy observed in the two vaccines. The MidaSpot rapid test detected 100% anti-SARS-CoV-2 RBD positivity for fully vaccinated subjects in both Pfizer and Moderna cohorts post full vaccination but turned negative greater than 90 days post-vaccination for 5.4% of subjects in the Pfizer cohort, whose quantitative anti-IgG were near the minimum levels of the group. Immune responses were found to vary greatly among vaccinees. Personalized longitudinal monitoring of antibodies could be necessary to assess the immunity duration of vaccinated individuals.