Humoral immune response after COVID-19 infection or BNT162b2 vaccine among older adults: evolution over time and protective thresholds. (preprint)

ABSTRACT

INTRODUCTION The objectives of this study were to assess the dynamics of the SARS-CoV-2 anti-RBD IgG response over time among older people after COVID-19 infection or vaccination and its comparison with speculative levels of protection assumed by current data. METHODS From November 2020 to October 2021, we included geriatric patients with serological test results for COVID-19. We considered antibody titre thresholds thought to be high enough to protect against SARS-CoV-2 infection 141 BAU/ml for protection/vaccine efficacy > 89.3%. Three cohorts are presented. A vaccine group (n=34) that received two BNT162b2/Comirnaty injections 21 days apart, a group of natural COVID-19 infection (n=32) and a third group who contracted COVID-19 less than 15 days after the first BNT162b2/Comirnaty injection (n=17). RESULTS 83 patients were included, the median age was 87 (81-91) years. In the vaccine group at 1 month since the first vaccination, the median BAU/ml with IQR was 620 (217-1874) with 87% of patients above the threshold of 141 BAU/ml. Seven months after the first vaccination the BAU/ml was 30 (19-58) with 9.5% of patients above the threshold of 141 BAU/ml. In the natural COVID-19 infection group, at 1 month since the date of first symptom onset, the median BAU/ml was 798 (325-1320) with 86.7% of patients above the threshold of 141 BAU/ml and fell to 88 (37-385) with 42.9% of patients above the threshold of 141 BAU/ml at 2 months. The natural infection group was vaccinated three months after the infection. Five months after the end of the vaccination cycle the BAU/ml was 2048 (471-4386) with 83.3% of patients above the threshold of 141 BAU/ml. DISCUSSION On the humoral level, this supports the clinical results describing the decrease in vaccine protection over time.