Surveillance studies of the influenza viruses circulating in Europe and other countries in 2007 and 2008 have revealed rates of resistance to oseltamivir of up to 67% among H1N1 viruses. In the present study, we examined 202 clinical samples obtained from patients infected with H1N1 virus in Japan in 2007 and 2008 for oseltamivir resistance and found that three were oseltamivir resistant (1.5%). The 50% inhibitory concentrations (IC 50 s), as measured by a sialidase inhibition assay with these drug-resistant viruses, were >100-fold higher than those of the nonresistant viruses (median IC 50 , 12.6 nmol/liter). The His274Tyr (strain N2 numbering) mutation of the neuraminidase protein, which is known to confer oseltamivir resistance, was detected in these three isolates. Phylogenetic analysis showed that one virus belonged to a lineage that is composed of drug-resistant viruses isolated in Europe and North America and that the other two viruses independently emerged in Japan. Continued surveillance studies are necessary to observe whether these viruses will persist.

To demonstrate the impact of influenza epidemics on pediatric hospital admissions, admissions that were attributable to influenza and respiratory syncytial virus (RSV) infection to the pediatric ward of an urban general hospital in Japan were followed-up during a 4-month period from December to March 1991 through 1998. During the 1997-1998 influenza type A (H3N2) epidemic, a diagnosis of influenza type A (H3N2) was made in 26.3% of all patients admitted aged 15 years or lower. During the peak of the epidemic, as many as 50-70% of the admissions were attributable to influenza type A (H3N2). In the seven winters from 1991 to 1988, 14.0% of all admissions were associated with infection with influenza virus (mean age 4.4 years), and 17.5% were due to RSV. More patients were admitted to hospital for influenza than RSV infection in three of the seven seasons. Among the patients with influenza, 74.5% of the cases were previously healthy children. Influenza and RSV infection are leading causes of pediatric hospital admissions during the winter. Effective methods of prophylaxis are needed not only for high-risk patients, but for healthy young children.

To demonstrate the impact of influenza epidemics on pediatric hospital admissions, admissions that were attributable to influenza and respiratory syncytial virus (RSV) infection to the pediatric ward of an urban general hospital in Japan were followed-up during a 4-month period from December to March 1991 through 1998. During the 1997-1998 influenza type A (H3N2) epidemic, a diagnosis of influenza type A (H3N2) was made in 26.3% of all patients admitted aged 15 years or lower. During the peak of the epidemic, as many as 50-70% of the admissions were attributable to influenza type A (H3N2). In the seven winters from 1991 to 1988, 14.0% of all admissions were associated with infection with influenza virus (mean age 4.4 years), and 17.5% were due to RSV. More patients were admitted to hospital for influenza than RSV infection in three of the seven seasons. Among the patients with influenza, 74.5% of the cases were previously healthy children. Influenza and RSV infection are leading causes of pediatric hospital admissions during the winter. Effective methods of prophylaxis are needed not only for high-risk patients, but for healthy young children.

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Our data suggest that current inactivated vaccine is highly effective for protection against influenza type A(H3N2) virus infection regardless of antigenic drift. In contrast, the protective efficacy obtained by vaccination may not be sufficient against influenza type B virus infection, and especially in young children, it does not offer protection.