Influenza A (H1N1)pdm09 virus caused more severe disease among both adults and children during the 2010-2011 influenza season than did influenza A (H3N2) and influenza B viruses, according to data from CDC researchers.
“Some have suggested that the morbidity and mortality due to the 2009 pandemic virus was less than that seen in previous seasons and pandemic years, and some studies did not find substantial differences in clinical outcomes between illness caused by the 2009 H1N1 virus and illness caused with previous seasonal influenza viruses,” Sandra Chaves, MD, MSc, medical director of the Influenza Hospitalization Surveillance Network in the influenza division of the CDC, told Infectious Disease News. “However, previous influenza seasons and pandemics were different from that which we experienced in 2009, when young people were disproportionately affected in terms of hospitalization and death.”
The researchers used surveillance data to examine associations between virus type and outcomes. From October 2010 to April 2011, there were 6,284 hospitalizations for laboratory-confirmed influenza in the hospitals of the network. Hospitalization rates were 10.8 per 100,000 population for H3N2 influenza, 7.2 per 100,000 population for influenza B and 5.6 per 100,000 population for H1N1 influenza. This analysis included 3,621 hospitalizations: 830 were children and 2,791 were adults.
Among children, hospital stays were longer and ICU admission was more frequent among those with 2009 H1N1. More children with 2009 H1N1 required mechanical ventilation, and discharge diagnoses of pneumonia were also more common among those with 2009 H1N1. Among adults, there was no difference in length of hospital stay according to influenza type, but more patients with 2009 H1N1 were admitted to the ICU. More patients with 2009 H1N1 also required mechanical ventilation, and acute respiratory distress syndrome was also more common among adults with 2009 H1N1.
Among both children and adults, 2009 H1N1 was independently associated with severe disease. Among children, severity was also associated with chronic metabolic disease and neuromuscular disorder. Among adults, severity was associated with chronic lung disease and neuromuscular disorder.
Chaves said that among adults who were hospitalized, early treatment with influenza antiviral drugs lowered the risk of more severe outcomes, lowering their risk of needing ICU admission by half. However, they were unable to determine the effectiveness of antiviral drugs among hospitalized children. She also said that prior to the 2009 pandemic, influenza antiviral drugs were rarely used.
“It is important to emphasize that influenza can be severe and identify groups at risk for complications,” Chaves said. “In that context, we hope that these data help increase awareness of influenza and the importance of monitoring the various strains of the virus, as they are constantly changing.”
Disclosure: The researchers report no relevant financial disclosures.