An analysis of data from influenza cases in the U.S. state of Wisconsin indicates individuals with 2009 H1N1 infections were younger than those with H3N2, and that the risk of most serious complications was not higher in adults or children with 2009 H1N1 compared with recent seasonal strains, according to a study in the Journal of the American Medical Association.

 

Edward Belongia, of the Marshfield Clinic Research Foundation, Marshfield, Wisc., and colleagues conducted a study to compare the characteristics of pandemic and seasonal influenza A infections occurring in a defined population. The study consisted of active surveillance with 30-day follow-up for influenza cases among children and adults living in a 14-zip code area in Wisconsin.

 

The researchers identified 2009 H1N1 influenza in 545 patients; 221 cases of seasonal H1N1; and 632 patients with H3N2 infection. The study found no significant differences by strain in the proportion of children or adults with any serious outcome (pneumonia or hospital admission) during the 30 days after onset.

 

"In summary, we found that children were disproportionately affected by 2009 H1N1 infection, but the perceived severity of symptoms and risk of serious outcomes (pneumonia or hospital admission) were not increased in children with 2009 H1N1 infection relative to seasonal influenza A viruses. This study demonstrates the benefit of ongoing active influenza surveillance in a defined cohort with standardized testing criteria and uniform collection of clinical and epidemiologic data. The use of consistent enrollment and testing procedures offers the opportunity to directly compare illness patterns and outcomes over multiple seasons, and these results complement information obtained through traditional public health surveillance systems," the authors wrote.