Flu hospitalizations climb as US season hits new heights

Flu hospitalizations across the country are still increasing, and at least by one metric the season has reached a height not seen since the 2009-10 pandemic, according to the latest data from the Centers for Disease Control and Prevention's (CDC's) FluView, which tracks national influenza surveillance.

In addition, despite elderly adults being the most hospitalized group, they often do not receive influenza tests, new research shows.

Hospitalized older adults

The overall hospitalization rate was 31.5 per 100,000 population this week, up from 22.7 last week. Adults 65 and older were most likely to be hospitalized, at a rate of 136.5 per 100,000 population, up from last week's rate of 98 per 100,000. Adults aged 50 to 64 followed, with a rate of 33.2 per 100,000 population, and children aged 4 years and younger had a rate of 22.8 per 100,000 population.

Despite being the most hospitalized demographic, adults aged 65 and older who are hospitalized with fever or respiratory symptoms during flu seasons are less likely to have a provider-ordered influenza test than younger patients, according to new research published in the Journal of the American Geriatrics Society.

In the study, only 28% of study participants had provider-ordered influenza testing. Patients who were tested were younger than those not tested (an average age of 58 years versus 66 years) and were more likely (71% versus 49%) to have influenza-like illness (ILI). Among all 1,422 patients in the study, who were from four Tennessee hospitals, the presence of ILI and younger age were independent predictors of provider-ordered testing.

As of late December, the CDC recommends that antivirals be given to all patients with probable flu infections, even in the absence of laboratory evidence.

H3N2 still predominant

Influenza A still far outnumbers influenza B strains, the CDC said today. Among 8,990 hospitalizations nationwide last week, 89.4% were associated with influenza A virus, and 10.1% with influenza B virus. Only 0.3% of hospitalizations were linked with co-infection of influenza A and B, and 0.2% with influenza virus for which the type was not determined, the CDC said.

Of all tested laboratory samples received by the CDC last week, 87.5% were influenza A and 12.5% were influenza B. Of the "A" strains that were subtyped, 90% were H3N2, with the rest being 2009 H1N1. Seasons in which the H3N2 strain dominates are typically more severe.

Usually, influenza B cases begin to rise as the flu season progresses, but there's no evidence of that shift yet. In Europe however, roughly half of laboratory-confirmed flu cases are caused by influenza B, according to information today from the European Centre for Disease Prevention and Control (ECDC).

Earlier this week, Dan Jernigan, MD, MPH, director of the influenza division at the CDC's National Center for Immunization and Respiratory Diseases, in a CDC Public Health Grand Rounds, said the 2014-15 was more severe than this season so far. He called the current season "moderate to severe."

But in one indicator of severity, the CDC reported 10 new pediatric flu deaths this week. So far this season there have been 30 pediatric deaths, most from influenza A. Only 3 pediatric deaths this year have been associated with influenza B infection, including 1 reported last week.

Increase in ILI outpatients

The proportion of outpatient visits for ILI was 6.2%, up from 5.8% last week. This surpasses the 6% number reached during the 2014-15 flu season. It also surpasses the other four post-pandemic seasons (2010-11 on). The rate of ILI outpatient visits peaked at 7.7% during the 2009-10 pandemic.

Flu is active across all of the United States, with widespread influenza activity reported in 49 states and Puerto Rico. Only Hawaii had local flu activity, the same as last week.

New York City, Puerto Rico, and 32 states experienced high flu activity: Alabama, Arizona, Arkansas, California, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Mississippi, Missouri, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, Ohio, Oklahoma, Oregon, South Carolina, South Dakota, Tennessee, Texas, Virginia, West Virginia, Wisconsin, and Wyoming. That is up from 26 states last week.

Nine states (Alaska, Colorado, Idaho, Iowa, Massachusetts, Minnesota, North Dakota, Pennsylvania, and Rhode Island) reported moderate activity, with Washington, D.C., and Connecticut, Michigan, New Hampshire, Utah, Vermont, and Washington experiencing low activity. Only Delaware, Maine, and Montana reported minimal activity.

See also:

Jan 19 CDC FluView

Jan 17 J Am Geriatr Soc study

Jan 19 ECDC report  

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