Positives are passed to additional tubes of cell culture. Initial typing is carried out by fluorescent antibody techniques. Subtyping of type A viruses are carried out by hemagglutination-inhibition. For specimens collected in the influenza vaccine efficacy study, specimens are also processed by real time PCR. By collecting data on vaccination use among participants, we will be able to control for vaccine use in our study analyses. Given the low rate of vaccination uptake among college age individuals, we do not expect vaccine use to be a significant confounder in our study design.