Introduction
Influenza A viruses are divided into subtypes on the basis of the
antigenicity of their surface glycoproteins, hemagglutinin (HA) and
neuraminidase (NA); 15 HA and 9 NA subtype viruses have been
isolated from birds, but only H1N1, H2N2, and H3N2 subtype
viruses have circulated widely and caused epidemics in humans
in the last century. Beginning in late 2003, outbreaks of H5N1
influenza A virus infection occurred among poultry and wild birds
in numerous Asian countries, with subsequent reports in Europe
and Africa (Office International des Epizooties [OIE]; http://
www.oie.int). Despite substantial infection control efforts, H5N1
viruses have continued to evolve and spread, producing human
infections in 15 countries, with 329 of the 562 confirmed cases
proving fatal (World Health Organization [WHO]; http://
www.who.int). These viruses thus pose a significant pandemic
potential to public health. In April 2009, an antigenically distinct
swine-origin H1N1 influenza A virus was detected in humans
(CDC, 2009). This novel H1N1 virus, referred to as the pandemic
2009 H1N1 virus (2009 H1N1), spread efficiently around the world,
leading the WHO to declare a global pandemic on June 11, 2009
(World Health Organization [WHO]; http://www.who.int).
Although infection with the 2009 H1N1 virus causes a mild, selflimiting
respiratory illness in most people, the young and those
with certain underlying conditions, including asthma, diabetes,
heart/lung problems, morbid obesity, and pregnancy, are at greater
risk of severe disease progression
IntroductionInfluenza A viruses are divided into subtypes on the basis of theantigenicity of their surface glycoproteins, hemagglutinin (HA) andneuraminidase (NA); 15 HA and 9 NA subtype viruses have beenisolated from birds, but only H1N1, H2N2, and H3N2 subtypeviruses have circulated widely and caused epidemics in humansin the last century. Beginning in late 2003, outbreaks of H5N1influenza A virus infection occurred among poultry and wild birdsin numerous Asian countries, with subsequent reports in Europeand Africa (Office International des Epizooties [OIE]; http://www.oie.int). Despite substantial infection control efforts, H5N1viruses have continued to evolve and spread, producing humaninfections in 15 countries, with 329 of the 562 confirmed casesproving fatal (World Health Organization [WHO]; http://www.who.int). These viruses thus pose a significant pandemicpotential to public health. In April 2009, an antigenically distinctswine-origin H1N1 influenza A virus was detected in humans(CDC, 2009). This novel H1N1 virus, referred to as the pandemic2009 H1N1 virus (2009 H1N1), spread efficiently around the world,leading the WHO to declare a global pandemic on June 11, 2009(World Health Organization [WHO]; http://www.who.int).Although infection with the 2009 H1N1 virus causes a mild, selflimitingrespiratory illness in most people, the young and thosewith certain underlying conditions, including asthma, diabetes,heart/lung problems, morbid obesity, and pregnancy, are at greaterrisk of severe disease progression
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