In conclusion, we demonstrated that vaccination against seasonal infl uenza may boost a cross-reactive immunity against an unrelated strain responsible for deadly infections in humans, i.e., the avian infl uenza (H5N1) strain A/Hong Kong/156/97. These data, together with previous experimental results from mice studies and epidemiologic reports, indicate that cross-type immunity should be considered an important component of the immune response against novel infl uenza A infections. Human Bocavirus Infections in Hospitalized Children and Adults Studies have reported human bocavirus (HBoV) in children with respiratory tract infections (RTIs), but only occasionally in adults. We searched for HBoV DNA in nasopharyngeal aspirates (NPAs) from adults with exacerbations of chronic bronchitis or pneumonia, from children hospitalized for acute RTIs, and from asymptomatic children during the winter of 2002–2003 in Canada. HBoV was detected in NPAs of 1 (0.8%) of 126 symptomatic adults, 31 (13.8%) of 225 symptomatic children, and 43 (43%) of 100 asymptomatic children undergoing elective surgery. Another virus was detected in 22 (71%) of the 31 HBoV-positive NPAs from symptomatic children. Two clades of HBoV were identified. The pathogenic role of HBoV in RTIs is uncertain because it was frequently detected in symptomatic and asymptomatic children and was commonly found with other viruses in symptomatic children. H uman bocavirus (HBoV) is a newly described human virus closely related to bovine parvovirus and canine minute virus. It is currently classifi ed in the genus Bocavirus within the family Parvoviridae. This virus was fi rst identifi ed in respiratory tract specimens from Swedish children with lower respiratory tract infections (RTIs) (1) . Nucleic acid amplifi cation has detected HBoV in respiratory samples of children with acute respiratory disease, with incidence rates ranging from 3% to 19% . However, the pathogenic role of HBoV is uncertain because other viruses have been frequently detected in HBoV-positive children with lower RTIs (range 37%-90%) (2, 3, 7, (9) (10) (11) (20) (21) (22) . The objective of this study was to describe the incidence and clinical manifestations of HBoV infections in children and adults with respiratory tract symptoms, including a control group of children without symptoms.