Acute tubulointerstitial nephritis after the second dose of the Pfizer COVID-19 vaccine.

Coronavirus disease 2019 (COVID-19) vaccine has been extended to children 6 months and older and boosters to those 12 years and older, and vaccine safety continues to be monitored. A 12-year-old female presented with non-oliguric acute kidney injury 6 days after receiving the second dose of Pfizer COVID-19 vaccine. Renal biopsy revealed idiopathic severe acute tubulointerstitial nephritis (TIN), which had a temporal relationship with the second dose of the COVID-19 vaccine. Patient received pulse methylprednisolone (30 mg/kg) intravenous for 3 days with improvement of kidney function, followed by prednisone at 60 mg oral daily for 6 weeks. After discontinuation of corticosteroids, kidney function worsened and treatment was initiated with mycophenolate mofetil (MMF). Kidney function improved after starting MMF. Patient remained on MMF for a year and then tapered. Kidney function remained normal without any proteinuria one year after MMF was discontinued. We present a case of acute TIN after a second dose of the Pfizer COVID-19 vaccine in the United States. As this adverse outcome is less frequently reported in the U.S. than other countries, does TIN have a stronger temporal (not causal) association with a certain COVID-19 vaccine or protocol? This has implications for future COVID-19 vaccine use.