Three-month follow-up of durability of response to the third dose of the SARS-CoV-2 BNT162b2 vaccine in adults aged 60 years and older: a prospective cohort study

Abstract

Objective To evaluate the durability of response 3 months after the third BNT162b2 vaccine in adults aged 60 years and older.Design Prospective cohort study.Setting Single tertiary centre.Participants Healthcare workers/family members aged ≥60 years old who received the third BNT162b2 dose.Interventions Blood samples were drawn immediately before (T0), 10–19 days (T1) and 74–103 days (T2) after the third dose.Primary and secondary outcome measures Anti-spike IgG titres were determined using a commercial assay and seropositivity was defined as ≥50 arbitrary units (AU)/mL. Neutralising antibody titres were determined at T2. Adverse events, COVID-19 infections and Clinical Frailty Scale (CFS) levels were documented.Results The analysis included 97 participants (median age, 70 years (IQR, 66–74), 58% CFS level 2). IgG titres, which increased significantly from T0 to T1 (median, 440 AU/mL (IQR, 294–923) and median, 25 429 AU/mL (IQR, 14 203–36 114), respectively; p<0.001), decreased significantly by T2, but all remained seropositive (median, 8306 AU/mL (IQR, 4595–14 701), p<0.001 vs T1). In a multivariable analysis, only time from the second vaccine was significantly associated with lower IgG levels at T2 (p=0.017). At T2, 60 patients were evaluated for neutralising antibodies; all were seropositive (median, 1294 antibody titres; IQR, 848–2072). Neutralising antibody and anti-spike IgG levels were correlated (r=0.6, p<0.001). No major adverse events or COVID-19 infections were reported.Conclusions Anti-spike IgG and neutralising antibody levels remain adequate 3 months after the third BNT162b2 vaccine in healthy adults aged ≥60 years, although the decline in IgG is concerning. A third dose of vaccine in this population should be top priority.