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 (T_0), 15 and 90 days after the second dose (T_1 and T_2), on the day of the third dose (T_3) and one month after the third dose (T_4).

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 T_1, that increased at T_2 and T_3, peaking at T_4.

Independent factors associated with a negative antibody titer at T_4 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.

Graphical abstract