Have we missed AINything? Acute interstitial nephritis in SARS-CoV-2 infection and vaccination

Acute interstitial nephritis (AIN), defined by the presence of interstitial inflammation accompanied by tubulitis, is an often overlooked cause of acute kidney injury (AKI). It is now well established that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection can cause a wide variety of kidney injuries, most commonly acute tubular injury and collapsing glomerulopathy. In comparison, AIN is rarely documented in association with SARS-CoV-2 both anecdotally and in larger series of autopsy or biopsy studies. In this issue of the Journal, León-Román describe five cases of AIN in patients with a history of coronavirus disease 2019 (COVID-19) and highlight AIN as a possibly under-reported or ignored facet of renal disease associated with SARS-CoV-2. They describe three scenarios in which AIN can be seen: (i) SARS-CoV-2 infection after diagnosis of AIN, (ii) AIN followed by SARS-CoV-2 infection in the same admission and (iii) Severe SARS-CoV-2 and AIN possibly associated with SARS-CoV-2 itself. Overall, AIN remains rare in SARS-CoV-2 and causality is difficult to ascertain. Interestingly, AIN is not only seen in association with the disease itself but also with SARS-CoV-2 vaccination. This scenario is equally rare and causality is no less difficult to prove. A history of preceding SARS-CoV-2 infection and vaccination should be actively sought when patients present with otherwise unexplained AIN.