In the JournalsPerspective

Influenza burden shifted during the 2009-2010 pandemic

The hospitalization rates caused by influenza during the 2009-2010 influenza pandemic were increased fivefold among children aged 5 to 17 years and sixfold among adults aged 18 to 49 years compared with previous seasons, recent data indicate.

“During typical influenza seasons, young children and elderly individuals experience the highest rates of influenza-related hospitalization,” the researchers wrote. “However, the majority of studies conducted in the northern and southern hemispheres during the pandemic have reported a high burden of disease among children and younger adults and a lower than expected disease burden among elderly individuals.”

Researchers with the Emerging Infections Program network conducted surveillance among 22.1 million people from April 15, 2009, to April 30, 2010. Those with laboratory confirmation of influenza virus were defined as cases. This analysis included those with confirmed or probable A(H1N1)pdm09 infection.

During the pandemic, there were 7,942 hospitalizations due to confirmed influenza; 7,717 were included in this analysis. The rate of pediatric influenza-associated hospitalizations was 44 episodes per 100,000 population. The rate of adult influenza-associated hospitalizations was 29 episodes per 100,000 population.

The highest rate of hospitalizations occurred in children aged younger than 5 years. Children aged younger than 6 months had a rate of 202 hospitalizations per 100,000 population, and children aged 6 to 23 months had a rate of 88 hospitalizations per 100,000 population. The lowest rate of hospitalizations occurred among adults aged 18 to 49 years, with a rate of 27 hospitalizations per 100,000 population.

In the previous six influenza seasons, the mean pediatric hospitalization rate was 16 episodes per 100,000 population. In the previous four influenza seasons, the mean adult hospitalization rate was 10 episodes per 100,000 population. The two groups with the highest increases were the 5- to 17-year-old group, in which the rate of hospitalization was 31 per 100,000 population vs. a previous average of six hospitalizations per 100,000 population, and the 18- to 49-year-old group, in which the rate of hospitalization was 27 per 100,000 population vs. a previous average of four hospitalizations per 100,000 population.

“We found evidence of a shift in disease burden when comparing pandemic-associated hospitalization rates to previous influenza seasons, including higher rates of hospitalization among school-aged children and younger adults and a lower rate among elderly individuals,” the researchers wrote. “Our results highlight the importance of surveillance in identifying risk factors for influenza hospitalization, monitoring adherence to influenza hospitalization, monitoring adherence to influenza prevention and treatment strategies and evaluating the disease burden during pandemic periods.”

Disclosure: One of the researchers has served as a consultant for Dynavax, GlaxoSmithKline, Merck, Novartis, Pfizer and Sanofi-Pasteur.

The hospitalization rates caused by influenza during the 2009-2010 influenza pandemic were increased fivefold among children aged 5 to 17 years and sixfold among adults aged 18 to 49 years compared with previous seasons, recent data indicate.

“During typical influenza seasons, young children and elderly individuals experience the highest rates of influenza-related hospitalization,” the researchers wrote. “However, the majority of studies conducted in the northern and southern hemispheres during the pandemic have reported a high burden of disease among children and younger adults and a lower than expected disease burden among elderly individuals.”

Researchers with the Emerging Infections Program network conducted surveillance among 22.1 million people from April 15, 2009, to April 30, 2010. Those with laboratory confirmation of influenza virus were defined as cases. This analysis included those with confirmed or probable A(H1N1)pdm09 infection.

During the pandemic, there were 7,942 hospitalizations due to confirmed influenza; 7,717 were included in this analysis. The rate of pediatric influenza-associated hospitalizations was 44 episodes per 100,000 population. The rate of adult influenza-associated hospitalizations was 29 episodes per 100,000 population.

The highest rate of hospitalizations occurred in children aged younger than 5 years. Children aged younger than 6 months had a rate of 202 hospitalizations per 100,000 population, and children aged 6 to 23 months had a rate of 88 hospitalizations per 100,000 population. The lowest rate of hospitalizations occurred among adults aged 18 to 49 years, with a rate of 27 hospitalizations per 100,000 population.

In the previous six influenza seasons, the mean pediatric hospitalization rate was 16 episodes per 100,000 population. In the previous four influenza seasons, the mean adult hospitalization rate was 10 episodes per 100,000 population. The two groups with the highest increases were the 5- to 17-year-old group, in which the rate of hospitalization was 31 per 100,000 population vs. a previous average of six hospitalizations per 100,000 population, and the 18- to 49-year-old group, in which the rate of hospitalization was 27 per 100,000 population vs. a previous average of four hospitalizations per 100,000 population.

“We found evidence of a shift in disease burden when comparing pandemic-associated hospitalization rates to previous influenza seasons, including higher rates of hospitalization among school-aged children and younger adults and a lower rate among elderly individuals,” the researchers wrote. “Our results highlight the importance of surveillance in identifying risk factors for influenza hospitalization, monitoring adherence to influenza hospitalization, monitoring adherence to influenza prevention and treatment strategies and evaluating the disease burden during pandemic periods.”

Disclosure: One of the researchers has served as a consultant for Dynavax, GlaxoSmithKline, Merck, Novartis, Pfizer and Sanofi-Pasteur.

    Perspective
    Theodore C. Eickhoff

    Theodore C. Eickhoff

    What makes this report worth publishing is the fact that the investigators, led by the Emerging Infections Program at CDC, have carried out population-based studies of rates of hospitalization due to laboratory-confirmed influenza in children and adults during the 2009-10 influenza pandemic. The Emerging Infections Network is a consortium of local and state health departments, academic institutions and medical “communities” that include 5.3 million children aged younger than18 years and 16.8 million adults aged 18 years and older. This amounts to about 7% of the US population. Of great interest is the fact that this program has gathered comparable data during preceding seasonal influenza outbreaks since 2003 for children and 2005 for adults.

    Thus, it is possible to calculate population-based rates of hospitalization quite accurately. Quite by chance, another report simultaneously appeared in Eurosurveillance (volume 17, issue 45)  describing hospitalizations associated with seasonal and pandemic H1N1 influenza from 2005-2010 in Hong Kong.  Their findings were quite comparable, although the age groupings differed slightly.

    It will be of great interest to report such data again 4-5 years hence — to see just how the pH1N1 virus behaves in the future.

    • Theodore C. Eickhoff, MD
    • Infectious Disease News Editorial Board member

    Disclosures: Dr. Eickhoff reports no relevant financial disclosures.