Among the HCWs we studied, working at hospital 2 conferred protection against pandemic (H1N1) 2009 virus infection. This hospital was in a geographic area with fewer cases than the others, but if this were the explanation, then a similar fi nding might have been expected in the nonclinical group, which was not demonstrated. Furthermore, at least as many cases of confi rmed pandemic (H1N1) 2009 infl uenza were seen at hospital 2 as were seen at the other hospitals (Table 1 ). Factors such as reported compliance with PPE, were adjusted for in the multivariable analysis to reduce the effect of hospital type on infl uenza risk. The reason for the lower risk associated with hospital 2 has not been identifi ed but may relate to other unmeasured factors, such as compliance with hand hygiene procedures.