H1N1 pandemic dominated 2009-2010 flu season

**CDC. MMWR. 2010;59:901-908.**

The 2009 H1N1 pandemic caused the highest rates of influenza-related pediatric hospitalization and mortality recorded since 1997, according to data from the CDC.

From April, 2009 through June 12, 2010 the number of laboratory-confirmed cases of influenza was four times the average of the previous four seasons, and the pandemic strain dominated, accounting for about 99% of influenza cases.

Transmission and detection of the H1N1 virus persisted from spring into summer, with levels of outpatient visits for influenza-like illness remaining elevated throughout the summer months. However, the levels did not exceed baseline until late summer and early fall of 2009, except for 1 week in April.

According to the report, the disease burden is not easily quantified due to the fact that many infected individuals do not seek medical care and influenza testing for the pandemic strain lacked sensitivity. During the 2009-2010 season in particular, point-of-care tests were not as sensitive for H1N1 as they were for seasonal influenza viruses, thereby possibly reducing the number of laboratory-confirmed cases.

According to a CDC-developed model, approximately 43 to 89 million people contracted the 2009 pandemic H1N1 virus between April 2009 and April 2010.

CDC researchers noted that in the first wave of the pandemic, from April through Aug. 29, 2009, 65 children died of laboratory-confirmed influenza. In the second wave, another 279 children died from influenza, CDC officials noted.

“Testing for seasonal influenza and monitoring for novel influenza virus infections should continue year-round, as should specimen submission to CDC for further antigenic analysis, vaccine strain selection and antiviral resistance monitoring,” the researchers wrote. “The detection of three novel influenza cases of swine-lineage H3N2 infection since July 2009 further emphasizes the importance of continuing to monitor for novel influenza strains.”

PERSPECTIVE

Last year's H1N1 influenza pandemic fortunately did not rival the 1918 pandemic in terms of overall morbidity and mortality, but it did cause a significant medical burden. Young children and those with underlying medical conditions were particularly affected. As we plan for the upcoming influenza season, we need to be mindful that influenza is a problematic foe. It is capable of changing its structure to evade preexisting immunity, of evolving from other mammalian or avian hosts and of spreading globally with great speed. Surveillance of influenza strains throughout the world must continue, and improved diagnostics, vaccines, and therapeutics are needed. In the meantime, all health care providers need to be immunized with influenza vaccine and they need to remind their patients to do the same.

– Kathryn M. Edwards, MD
Infectious Diseases in Children Editorial Board member