*Purpose: Vaccination generates immune memory, including antibodies against harmful pathogens. Transplant immunosuppression prevents similar processes occurring against alloantigens. Transplant recipients have significantly reduced seroresponses to vaccination. We hypothesized that in renal transplant recipients, seroconversion to vaccination may predict future rejection episodes while a lack of seroconversion might predict subsequent infection.

*Methods: We followed 151 patients who received the 2009 monovalent pandemic H1N1 vaccine at our institution, until December 2016. Patients were considered seroconverters (n=48) or non-seroconverters (n=103) (seroconversion defined as a ≥4-fold increase in antibody titre). Time to first rejection or serious infection (infection requiring hospitalisation) was compared between seroconverters and non-seroconverters using Cox regression. In addition, prior rejection or serious infection were assessed by logistic regression as predictors of seroconversion. Regression models were adjusted for age, sex, transplant duration, peak PRA, eGFR and mycophenolate dose. eGFR, patient and graft survival were also examined. Registry (ANZDATA) and medical record data was used.

*Results: The cohort had a mean age of 51.2±12.6 years, 55.6% were male, 91% had first transplants. The median time post-transplant was 3.7 years (range 0.1-27.9 years). Immunosuppression consisted of a calcineurin inhibitor in 90%, mycophenolate in 81.5% and prednisolone in 63.6%. Twenty-six rejection episodes occurred in the follow up period. Seroconversion was not associated with an increased risk of rejection on univariable or multivariable analysis (HR 1.20, 95% CI 0.50-2.83, p=0.678). Fifty-eight patients developed a serious infection in the follow-up period. Seroconversion was not associated with subsequent serious infection on univariable or multivariable analysis (HR 0.94 95% CI 0.50-1.75, p=0.835). Seroconversion did not predict patient or graft survival and using a mixed model, no difference in eGFR at 5 years post-vaccination was apparent between seroconverters and non-seroconverters (coefficient -0.38, CI -1.02-0.27, p=0.256). A lower proportion of patients with a rejection episode prior to vaccination seroconverted relative to those without prior rejection (22% vs 39%, p=0.029), however this association was not significant on multivariable analysis OR 0.51, 95%CI 0.23-1.14, p=0.10. Serious infection prior to vaccination was not associated with seroconversion (OR 1.28, 95%CI 0.47-3.51, p=0.63).

*Conclusions: Despite a compelling premise we were unable to detect a significant association between vaccine response and the immunologic events of rejection or infection post-vaccination. The univariable association of reduced seroconversion with prior rejection may reflect an increased immunosuppressive load and reduced eGFR as the association was lost after adjustment for these factors. It will be of interest to examine whether seroresponses to other vaccine types, such as polysaccharide vaccines, are more valuable in predicting immune events.

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