Tackling Trends in This Year's Flu Season

Flu season H1N1 infections tied to more hospitalization of younger adults than previous years

/ Author:  / Reviewed by: Robert Carlson, M.D Beth Bolt, RPh

(RxWiki News) Flu season may be largely over in the US, but that only means it's time to prepare for next year. A new report considered this year's trends and next year's chances.

The report, by researchers from the Centers for Disease Control and Prevention (CDC), analyzed data on US flu cases during the 2013 to 2014 season.

The report found that flu infections peaked in late December, and more adults between the ages of 50 and 64 were hospitalized than in the last several years.

"Remember to cover your mouth when coughing or sneezing."

The authors of this new report, which was led by Scott Epperson, MPH, of CDC's National Center for Immunization and Respiratory Diseases, utilized data from the World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the US.

This included 308,471 samples tested for influenza during September 29, 2013 until May 17, 2014. Of these, 53,470 (17.3 percent) were positive for the flu virus.

Most of the positive samples (87.4 percent) were influenza A strains, and 12.6 percent were influenza B. Most of the 31,353 influenza A samples that were further analyzed were of the H1N1 strain (90.3 percent).

H1N1 is a swine flu strain that caused many illnesses in 2009. This year's season was the first since 2009 to see flu infections that were predominately caused by this strain.

Dr. Epperson and team noted that this season had lower overall illness and death rates than in previous seasons in which different strains were more common, but that hospitalization rates among younger adults were the highest they had been in several years.

For example, during October 1, 2013 to April 30, 2014, the researchers estimated that 54.3 people per every 100,000 adults between the ages of 50 to 64 were hospitalized. During the previous four flu seasons, this number has ranged from 8.1 hospitalizations to 40.6 hospitalizations per every 100,000 adults aged 50 to 64.

During this same time period of October through April, 2014, adults between the ages of 18 and 64 accounted for 57.4 percent of the flu hospitalizations that were reported.

Dr. Epperson and team noted that influenza infection numbers across the country (or "flu activity") increased through November and December of 2013, before peaking during late December—specifically, the week that ended on December 28.

However, some variations were seen regionally, as flu activity in the South peaked early, during the week that ended on December 7, while flu activity in the East peaked late, during the week that ended January 25.

After the flu activity "peaks," infection numbers started to drop off.

The researchers noted it has been recommended that next year's flu vaccine for the 2014-2015 season should be similar to the one used during the 2013-2014 season. This vaccine formula aims to protect against a number of strains, including H1N1.

The study was published online June 6 in CDC's Morbidity and Mortality Weekly Report. No conflicts of interest were reported. 

Review Date: 
June 7, 2014
Last Updated:
June 9, 2014