How did the epidemiology of the 2009-2010 H1N1 ("swine flu") influenza differ from typical influenza seasons?

Updated: Aug 07, 2020
  • Author: Hien H Nguyen, MD, MS; Chief Editor: Michael Stuart Bronze, MD  more...
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In contrast to typical influenza seasons, the 2009-2010 influenza season was affected by the H1N1 (“swine flu”) influenza epidemic, the first wave of which hit the United States in the spring of 2009, followed by a second, larger wave in the fall and winter; activity peaked in October and then quickly declined to below baseline levels by January, but small numbers of cases were reported through the spring and summer of 2010. [22]

In addition, the effect of H1N1 influenza across the lifespan differed from that of typical influenza. Disease was more severe among people younger than 65 years than in nonpandemic influenza seasons, with significantly higher pediatric mortality and higher rates of hospitalizations in children and young adults. Of the 477 reported H1N1-associated deaths from April to August 2009, 36 were in children younger than 18 years; 67% of those children had 1 or more high-risk medical conditions. [22]

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