JC and Human polyomavirus 9 after kidney transplantation: An exploratory serological cohort study

•15% and 8% of kidney tx recipients experienced increased IgG levels against JCPyV and HPyV9, respectively.•Healthy individuals did not experience large fluctuations in polyomavirus IgG levels.•Recipient JCPyV and HPyV9 virus IgG levels were respectively correlated with donor JCPyV IgG levels and HPyV9 IgG levels.•The kidney transplant may represent the source of JCPyV and HPyV9s infection after KTx.