The week that included Christmas saw influenza activity increasing markedly in the United States, with 43 states reporting widespread flu, the Centers for Disease Control and Prevention (CDC) said in its weekly FluView update today.
In addition, the CDC reported 6 new pediatric deaths, bringing the season's total to 21.
The agency has predicted that this season will likely be more severe than average because of dominance of the H3N2 strain and a mismatch between the circulating H3N2 strain and the one in the flu vaccine.
Only Maine and some western states, including Alaska, reported regional activity; the rest had widespread influenza. The number of states reporting widespread flu grew from 36 states the week before to 43. In addition, 29 states experienced high influenza-like illness (ILI) activity, up from 22 the week before.
The proportion of people seeking care for ILI grew from 5.5% to 5.9%, well above the national baseline of 2%. This marked the sixth consecutive week the country has been above baseline for that flu marker.
The proportion of respiratory samples that tested positive for flu also increased, from 28.1% the week before to 30.4% for the week ending Dec 27.
Flu hospitalizations, deaths
The season's cumulative rate of flu-related hospitalizations climbed to 12.6 per 100,000 population, up from 9.7 the week before.
Not surprisingly, the highest rate of flu-related hospitalization was in those 65 and older. The rate in that age-group was 51.8 per 100,000 people in the most recent week, up dramatically from 38.3 the week before.
In its summary of the FluView data, the CDC noted that the hospitalization rate in the elderly in the same week in 2012-13—the previous H3N2-dominated season—was 52.8 per 100,000.
Of the 6 influenza-related pediatric deaths, 3 were caused by H3N2, 2 were caused by an influenza A virus that was not subtyped, and 1 was caused by an undetermined strain. All occurred in the last 2 weeks of 2014.
The proportion of deaths due to flu and pneumonia stayed level, at 6.8%, just below the US epidemic threshold of 6.9%.
H3N2 dominance, mismatch
Of the 7,289 viruses that tested positive for flu during the most recent week, 7,041 (96.6%) were influenza A, with the rest influenza B. Of the 1,641 "A" viruses that were subtyped, 1,635 (99.6%) were H3N2 and 6 (0.4%) were 2009 H1N1.
Of the H3N2 viruses that were antigenically or genetically characterized, 68.3% were mismatched with the H3N2 strain in the vaccine. That percent climbed from 67.4% the week before.
None of the 295 H3N2, 11 2009 H1N1, or 80 influenza B viruses that CDC labs have tested so far have proved resistant to the commonly used antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza).
Jan 5 CDC FluView update
Jan 5 CDC summary of FluView data