ABSTRACT
Introduction: Messenger RNA (mRNA) based vaccines (Pfizer/BioNTech and Moderna) have shown to be highly effective at providing immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Aims: To study the duration of immunity, evolution of IgG antibody levels and IgG avidity (an index of antibody-antigen binding strength), and differences in immune responses between (mRNA) based vaccines. Methods: We performed a prospective study of 72 subjects without a history of SARS-CoV-2 who received both doses of either the Pfizer (n=55) or Moderna (n=17) mRNA vaccine. Anti-spike protein receptor binding domain (RBD) IgG antibody levels and IgG avidity indices were measured longitudinally using a qualitative finger stick MidaSpot™ rapid test at the point-of-care and a quantitative dry blood spot-based pGOLD™ laboratory test over ∼ 4 months post-vaccination. Results: On average, anti-RBD IgG antibody levels peaked at ∼2 weeks after second vaccination dose and declined thereafter, while antibody avidity increased suggesting antibody maturation (Figure). Moderna vaccine recipients exhibited higher side effect severity, higher peak anti-RBD IgG antibody levels and higher avidity up to the 90 days point, when compared to Pfizer vaccine recipients. Nevertheless, the differences in antibody and avidity levels diminished at ∼ 120 days post-vaccination, in line with the similar efficacy of the two vaccines. A qualitative MidaSpot finger stick rapid test detected 100% anti-SARS-CoV-2 RBD positivity for fully vaccinated subjects in both Pfizer and Moderna cohorts and turning negative greater than 90 days post-vaccination for 9% of subjects in the Pfizer cohort whose quantitative anti-IgG fell below the 25-percentile levels. Conclusions: Longitudinal quantitative measurements of anti-RBD antibody and avidity levels provide insight to immune responses and could aid the assessment of immunity and vaccine effectiveness.