To the Editor We read with interest the article by Downes et al1 regarding the risk of acute kidney injury (AKI) in hospitalized children treated with vancomycin and piperacillin-tazobactam (TZP).1 The adjusted odds ratio for AKI in this population drawn from 6 large children’s hospitals was 3.40 (95% CI, 2.26-5.14). This estimate is very similar to the results of a 2017 meta-analysis including studies of both children and adults.2 In the comparison group treated with vancomycin plus an alternative β-lactam agent, the risk of AKI was 4.4% (40 of 906 patients). Working from the adjusted odds ratio estimate, we calculated an absolute increase in risk of 9.55% for vancomycin plus TZP vs alternative regimens. This results in an estimate of the number needed to harm of 10.5. Using the lower bound of the 95% confidence interval, the number needed to harm is 19.4; using the upper bound, the number needed to harm is 5.2.