US flu has likely peaked, but hospital illness still high

The percentage of outpatient visits for influenza-like illnesses (ILI) dropped for the first time this season to 6.4%, a percentage point down from last week's 7.5%, one of a handful of indicators in the Center for Disease Control and Prevention's (CDC's) weekly FluView surveillance data that flu may finally be on the decline during this long season.

Two weeks ago, outpatient ILI visits were at 7.7%, a percentage not seen since the 2009 H1N1 pandemic. This year is not a pandemic year, but has instead been dominated by H3N2, an influenza A subtype that is notoriously difficult for the flu shot to protect against. Though the drop to 6.4% is significant, the CDC warned that the percentage "remains higher than the peak of flu activity observed during many seasons."

In another encouraging sign of a possible decline in flu activity, the number of states reporting high ILI dropped from 43 to 39, but 48 states and Puerto Rico are still reporting widespread flu.

Flu-related hospitalizations, however, continue at very high levels.

Pediatric deaths, hospitalization rates climb

Several indicators, in fact, still show the danger and severity of this year's season. The CDC confirmed 13 new pediatric deaths, bringing the season's total to 97.

"CDC continues to recommend influenza vaccination for all persons 6 months of age and older as flu viruses are likely to continue circulating for weeks and there is an increasing proportion of influenza B and H1N1 viruses being detected," the CDC said in its FluView summary.

Preliminary data released 2 weeks ago shows that children have better protection rates against the flu after getting vaccinated, around 50%. Moreover, the flu shot offers substantial protection against H1N1, and moderate protection against influenza B, the CDC said.

Hospitalizations also increased this week, to a cumulative overall rate of 74.5 hospitalizations per 100,000 people in the United States. Last week that number was 67.9. People over the age of 65 are the most likely to be hospitalized at a rate of 322.7 per 100,000, followed by adults aged 50 to 64 years (79.9 per 100,000), and younger children aged 0 to 4 years (52.6 per 100,000). All of those numbers increased from the previous report.

"These rates are higher than the end-of-season rates for the 2014-2015 season," the CDC said. "For that season, overall hospitalization was 64.2 per 100,000; hospitalization rates for people 65 years and older were 308.8 per 100,000. The hospitalization rate for people 50-64 years was 53.4 per 100,000. The hospitalization rate for children 0-4 years were 57.3 per 100,000."

The 2014-15 season has been a benchmark for this year—it was another long, non-pandemic flu season dominated by H3N2.

Influenza B, H1N1 both rise

Though influenza A is still the dominant strain this season, there was a substantial increase in influenza B cases, typical of this time of year. Of all positive lab samples reported to the CDC last week, 58.2% were influenza A viruses and 41.8% were influenza B.

Of the subtyped influenza A cases, 73.7% were H3N2, with 22.5% H1N1. Yamagata lineage subtype made up the majority (70.2%) of influenza B cases.

Hospitalizations were still greatly associated with influenza A infection; 83.4% of all hospitalized flu cases were caused by influenza A.

See also:

Feb 23 CDC FluView

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