Influenza Hospitalizing Twice as Many as Last Year

December 30, 2014

Predictions of a harsher influenza season than usual are coming true, as the rate of influenza-related hospitalizations as of December 20 was more than double that a year ago, the Centers for Disease Control and Prevention (CDC) reported last week.

Influenza officially qualified as an epidemic on the week ending December 20, week 51 of 2014, because the proportion of deaths attributed to pneumonia and influenza increased to the epidemic threshold of 6.8%, according to the CDC's weekly FluView update.

Earlier this month, CDC Director Thomas Frieden, MD, MPH, cited two reasons why influenza may hit especially hard in the season that began on October 1. The first is that A(H3N2), the principal virus in circulation, typically is more virulent than others, causing twice as many hospitalizations and deaths. Compounding this bad draw of viruses is a recent genetic mutation or drift in the strain of A(H3N2) virus chosen for the 2014 to 2015 vaccine, which makes the shot less effective.

From October 1 through December 20, influenza accounted for a cumulative 9.7 hospitalizations per 100,000 population, according to FluView. That is more than double the hospitalization rate of 4.3 for the same period in the 2013 to 2014 season, when the 2009 pandemic A(H1NI) virus predominated, and 76% higher than the rate of 5.5 in 2012 to 2013, which was another A(H3N2) season.

The influenza strain this year is tougher than its predecessors in the previous two seasons by another measure: the proportion of outpatient visits for influenza-like illness (ILI). It was 5.5% as of December 20 and above the national baseline of 2% for the fifth consecutive week. In contrast, the ILI rate was only 3% in week 51 of 2013 and 4.2% in 2012.

In terms of influenza-associated hospitalizations and ILI, the A(H3N2) virus of 2014 to 2015 is more severe than its predecessor of 2012 to 2013. That is also true for pneumonia and influenza mortality, but less markedly so. The death rate of 6.8% in week 51 of 2014 was just slightly higher than the 6.5% posted in week 51 of 2012 (as well as the 6.7% for the same week in 2013).

However, the 15 pediatric deaths associated with influenza from October 1 to December 20 in 2014 are running slightly behind the 16 deaths reported for the same period in 2012.

Overall, 36 states experienced widespread influenza activity during the week ending December 20, up from 29 states the week before. Twenty-two states had high levels of ILI, for an increase of nine states.

The latest FluView report is available on the CDC website. The next report is scheduled for release on January 2, 2015.

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