Transmission of 2009 pandemic influenza A (H1N1) at a Public University--Delaware, April-May 2009.

We investigated the first documented university outbreak of the 2009 pandemic influenza A(H1N1) to identify factors associated with disease transmission. An online student survey was administered to assess risk factors for influenza-like illness (ILI), defined as fever with cough or sore throat. Of 6049 survey respondents, 567 (9%) experienced ILI during 27 March to 9 May 2009. Studying with an ill contact (adjusted risk ratios [aRR], 1.29; 95% confidence intervals [CI], 1.01-1.65) and caring for an ill contact (aRR, 1.51; CI, 1.14-2.01) any time during 27 March to 9 May were predictors for ILI. Respondents reported that 680 (6%) of 11,411 housemates were ill; living with an ill housemate was a predictor for ILI (RR, 1.38; CI, 1.04-1.83). Close contact or prolonged exposures to ill persons were likely associated with experiencing ILI. Self-protective measures should be promoted in university populations to mitigate transmission.