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Influenza in US hits epidemic threshold

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Influenza activity in the United States has reached an epidemic level for the 2014-2015 season, with national incidence of influenza-like illness quickly approaching the peak level seen in the 2012-2013 season, according to the CDC’s FluView report.

In the week ending Dec. 20, the proportion of people seeing a health care provider for influenza-like illness (ILI) increased to 5.5%, and is above the national baseline of 2% for the fifth consecutive week. All of the US regions reported ILI activity at or above regional baseline levels. Puerto Rico and 22 states experienced high ILI activity, compared to 13 states in the previous week. Widespread influenza activity was reported in 36 states.

There have been 2,643 laboratory-confirmed influenza-associated hospitalizations reported to the Influenza Hospitalization Surveillance Network since Oct. 1, for a cumulative rate of 9.7 hospitalizations per 100,000 population. The hospitalization rate among people aged 65 years and older was 38.3 per 100,000. The proportion of deaths related to pneumonia and influenza was at the epidemic threshold of 6.8%. There were four influenza-related pediatric deaths during the week ending Dec. 20, and a cumulative total of 15 pediatric deaths related to influenza this season.

Among the 6,152 positive influenza tests reported during the week of Dec. 20, 5,987 (97.3%) were influenza A viruses. Among the 2,023 influenza viruses that were subtyped, 99.9% were influenza A(H3) viruses. Of the 239 influenza A(H3N2) viruses that were characterized antigenically or genetically by the CDC, 161 (67.4%) were different from the H3N2 component of the Northern Hemisphere influenza vaccine. Most were similar to A/Switzerland/9715293/2013, the H3N2 component of the Southern Hemisphere influenza vaccine.

“Flu activity is expected to continue in the coming weeks, with increases occurring especially in those states that have not yet had significant activity,” CDC wrote in the report. “CDC continues to recommend vaccination as long as flu viruses are circulating.”