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Amantadine resistance has become increasingly common among recent seasonal influenza viruses. In addition, almost all pandemic (H1N1) 2009 viruses are resistant to this drug. Between 2007 and early 2009, the prevalence of oseltamivir-resistant seasonal H1N1 variants that possess the His274Tyr neuraminidase mutation had dramatically increased, raising concerns regarding the utility of oseltamivir. Fortunately, most of the pandemic (H1N1) 2009 viruses were sensitive to neuraminidase inhibitors. In this paper, drug resistance among recent influenza virus isolates is comprehensively reviewed, especially from the standpoint of the molecular basis of drug resistance.

Currently there have reported pandemic (H1N1) 2009 over 200 countries and areas globally with more than 16,813 victims according to World Health Organization. Demographic characterization of those cases seemed to be consistent around the world that is higher incidence among school age children and higher mortality rate among elderly. Uniquely Japan has lower mortality rate compared with other countries. Based upon pandemic preparedness plan, several interventions such as personal hygiene and social distancing as well as clinical management were implemented since the beginning of pandemic. Of particular, school closure was continuously implemented during pandemic and medical access for influenza like illness cases was sustained with administration of antivirals in Japan. It is important to identify some gaps in the implementation of interventions for further preparedness.

A new pandemic influenza A (H1N1) virus had emerged and rapidly spread throughout the world. The clinical pathological observations associated with severe cases of pandemic (H1N1) 2009 are similar to that of high pathogenic avian influenza (H5N1). In order to find the most effective treatment methods for this pandemic influenza (H1N1), we describe our experiences, investigations and collaboration studies of avian influenza (H5N1) in Vietnam in association of our cooperative study of pandemic (H1N1) 2009 in Mexico. Effective treatment methods for critical illness due to influenza will be discussed from medical, regional and global points of view, which may be applied for the treatment of any type of influenza virus.

Pandemic influenza H1N1 2009 caused more severe diseases in children and young adults than seasonal influenza. The typical manifestation of severe diseases was diffuse viral pneumonia complicated with acute respiratory distress syndrome. Over half of the severe cases had underlying conditions, in which pregnancy and morbid obesity were regarded as major risk factors. Pathological findings revealed that the virus efficiently replicated in alveolar cells and caused diffuse alveolar damage as well as bronchiolitis. The level of plasma cytokines was also elevated in the severe cases. Antiviral use was generally recommended for the severe cases and might improve the outcomes. Further studies are needed for better clinical management of the severe cases of the latest pandemic influenza.

In the spring of 2009, a novel swine-origin H1N1 virus, whose antigenicity is quite different from those of seasonal human H1N1 strains, emerged in Mexico and readily transmitted and spread among humans, resulting in the first influenza pandemic in the 21st century. Molecular analyses of the pandemic H1N1 2009 viruses indicate low-pathogenic features for humans, although worldwide transmission of the virus and a considerable numbers of lethal cases with acute pneumonia have been observed in the first wave of the current pandemic. Here, we review our current molecular knowledge of transmissibility and pathogenicity of influenza viruses and discuss the future aspects of the pandemic virus.

In the spring of 2009, a novel swine-origin H1N1 virus, whose antigenicity is quite different from those of seasonal human H1N1 strains, emerged in Mexico and readily transmitted and spread among humans, resulting in the first influenza pandemic in the 21st century. This novel H1N1 virus was shown to be a triple reassortant comprising genes derived from avian, human, and swine viruses. Here, we review our current knowledge of this pandemic influenza virus and discuss future aspects of the pandemic.

In the past, influenza pandemics have been occurring every 20 to 30 years. Highly pathogenic avian influenza A(H5N1) has been causing unprecedented global outbreaks since 2003 and many human cases with a high case fatality rate have also been reported. But the virus that caused a pandemic in 2009 was A(H1N1) that was originated from swine influenza. The same subtype, A(H1N1) has been circulating in human population since 1977. This pandemic (H1N1) 2009 is also not as virulent as A(H5N1) in humans. Many aspects of pandemic (H1N1) 2009 are different from what we had been expecting. We should reconsider the concepts and the strategies for influenza pandemic by reviewing current pandemic (H1N1).