Data from California indicate that there have been more severe flu cases this season than in any season since the 2009 H1N1 pandemic, and two-thirds of severe cases have occurred in those aged 41 to 64.
Middle-aged adults have been more likely to be hospitalized for and die from flu this season than they have been during the past 3 seasons, according to a report published in the February 21, 2014, issue of the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report
. The report
, which details flu activity for the 2013-14 influenza season in California, also found more severe cases occurring earlier in this season than in any other season since 2009.
As of January 18, 2014, 94 flu-related deaths and 311 admissions to the intensive care unit (ICU) were reported by local health jurisdictions in California, for a total of 405 severe cases, more than any season since the 2009 pandemic. A total of 103 severe cases were reported during the week ending January 11, 2014, representing the highest number of severe cases at this point in the season and the largest number of new cases occurring in a single week since 2009.
During the H1N1 flu pandemic in 2009, higher rates of hospitalization and death occurred in younger and middle-aged adults. For the first time since the pandemic, the pH1N1 virus is again the predominant circulating influenza virus this season, causing more severe cases among younger adults, the report suggests. Overall, 66% of severe cases occurred among patients aged 41 to 64 years. In addition, 77% of flu-related deaths and 63% of ICU admissions occurred within this age group. These rates are higher than any season from 2009 to 2013. Adults aged 41 to 64 were 6 times more likely to die and almost 4 times more likely to be admitted to the ICU due to flu than those aged 40 and younger.
“The reason for this difference is unknown and might include virologic factors or a relative lack of population immunity in this age group because of low rates of either vaccination or prior exposure,” an editorial note included in the report suggests.
Although rates were unusually high among middle-aged adults, the number of severe flu cases occurring among young children has been particularly low this season. Of 25 severe cases reported among children, only 6% occurred in those aged 4 years and younger, representing the lowest rates for this age group since the 2009 season.
The report also indicates that a majority of patients who have suffered flu-related deaths so far this season had a comorbid condition and had not received the flu shot. Among fatal cases with enough medical information to identify a comorbid condition, 93% had a condition rendering them more susceptible to severe influenza. Among patients with known vaccination status, just 21% had been vaccinated against the flu at least 2 weeks before their symptoms began.
“Clinicians should make influenza vaccination a priority for all patients, and early diagnosis and treatment of influenza-like illness should be a priority in the care of patients with preexisting conditions recognized by [the Advisory Committee on Immunization Practices] as increasing the risk for influenza complications,” the editorial note concludes.
With weeks and potentially months remaining in this flu season, vaccination is still recommended, especially for those at an increased risk for complications.