Introduction: The immune response to the primary (two-dose) series of mRNA COVID-19 vaccines in kidney transplant recipients (KTRs) is very weak. We conducted a longitudinal observational study to compare the humoral response to a third, additional primary dose of mRNA vaccines between infection-naïve (IN-KTRs) and previously infected KTRs (PI-KTRs). Methods: We measured the levels of anti-spike (anti-s) IgG antibodies before and 14-21 days after the third dose and, in the secondary analysis, we compared the antibody response to BNT162b2 versus mRNA-1273. The reactogenicity assessment included solicited local and systemic reactions. Results: A total of 112 KTRs were enrolled, including 83 IN-KTR and 29 PI-KTR, among whom seroconversion in anti-s antibodies after the primary two-dose vaccination was achieved in 45.78% and 100% of cases, respectively. After three months, a waning antibodies titer by 67.4% (IN-KTR) and 7.5% (PI-KTR) was observed. After the third dose of the mRNA vaccine, 71.08% (59/83) of IN-KTR and 96.5% (28/29) of PI-KTR samples were seroconverted with a median anti-s titer of 468.0 (195.0-1620.0) BAU/mL and 1629.0 (1205-1815) BAU/mL, respectively. Of those IN-KTR in whom the primary vaccination failed, 46.67% (21/45) of patients achieved seroconversion after the third dose. No serious adverse events after the third dose were reported. In strata analyses, after the third dose, 66% (40/60) of patients vaccinated with BNT162b2 and 82.6% (19/23) of patients vaccinated with mRNA-1273 seroconverted with a median anti-s titer of 384.5 (144-837) BAU/mL and 1620 (671-2040) BAU/mL, respectively. Conclusions: The use of a third dose of mRNA vaccine may be of benefit for KTR, especially for those in whom the primary vaccination failed. Vaccines with a higher dose of mRNA and a longer interval between doses of the primary vaccination, such as mRNA-1273, seem to be the preparations of choice in immunocompromised individuals.
Keywords: COVID-19; kidney transplant recipients; mRNA vaccines; seroconversion.