Kinetics of humoral immune response and severity of infection after three doses of SARS-CoV-2 mRNA vaccine in a large cohort of kidney transplant recipients

Background: COVID-19 in kidney transplant recipients is associated with high morbidity and mortality. In this study we aimed to evaluate: (i) the seroconversion rate after BNT162b2 (Pfizer-BioNTech) SARS-CoV-2 vaccine, (ii) factors associated with humoral response, (iii) clinical outcome of COVID-19 in kidney transplanted patients. Methods: We enrolled a cohort of 743 kidney transplant recipients followed up from March 2020 until April 2022. A subset of 336 patients, who received three-doses of SARS-CoV-2 vaccine, was analyzed in terms of kinetics of humoral immune response and compared to a control group of 94 healthcare workers. Antibody response was tested before vaccination (T0), 15 and 90 days after the second dose (T1 and T2), on the day of the third dose (T3) and one month after the third dose (T4). Results: We observed that 66 out of 743 subjects had COVID-19 infection pre-vaccination: 65.2% had severe symptoms, 27.3% were hospitalized (9 deaths), none were asymptomatic. After three doses, 51 patients had COVID-19 infection, 60.8% were asymptomatic, 27.5% reported mild symptoms, 3.9% showed severe symptoms, 7.8% were hospitalized (2 deaths). In the subset of 336 vaccinated patients, an antibody level > 0.8 U/ml was detected at T1, that increased at T2 and T3, peaking at T4. Independent factors associated with a negative antibody titer at T4 were decreasing estimated glomerular filtration rate, time from transplantation, and antimetabolites (all p < 0.001) and age (p = 0.007). Conclusions: The kinetics of humoral response after three doses of vaccine in kidney transplant patients is characterized by a late but effective immune response against SARS-CoV-2, reducing morbidity and mortality.