Flu activity drops across country as season ebbs

According to the Centers for Disease Control and Prevention's (CDC) latest FluView surveillance report today, influenza-like illness (ILI) activity is markedly down across the country this week, a clear sign that this year's severe flu season continues to wind down.

The percentage of outpatient visits for ILI was 3.7%, down from 5.0% the previous week. The national baseline is 2.2%. The current ILI rate is similar to what was observed at the height of the 2015-16 season.

Season not over yet

Despite the good ILI news, Anne Schuchat, MD, acting director of the CDC, said yesterday that more cases can be expected this season.

"We cannot predict how long this season will last, and while we have started to see a decline in rates of people visiting their doctor for influenza-like illness, we expect to see several more weeks of ongoing flu activity, with continued reports of hospitalizations and flu deaths in children and adults," Schuchat said as she testified at a hearing to the Subcommittee on Oversight and Investigation on this year's influenza season.

The number of states experiencing high levels of influenza activity also dropped, to 21 from the previous week's 32. Fifteen states had moderate flu activity, 5 states experienced low activity, and 9 states reported minimal activity.

Flu was geographically widespread in 34 states and Puerto Rico. Last week the CDC reported widespread flu in 45 states and Puerto Rico.

There were five pediatric deaths recorded, fewer than in previous weeks. So far this season the CDC has confirmed 119 pediatric deaths.

Influenza B and A now even

The rate of flu-related hospitalizations across the country, however, rose somewhat. The overall rate was 86.3 per 100,000 population, up slightly from the previous week's rate of 81.7 per 100,000 people.

"The overall hospitalization rate and all age-specific hospitalization rates are now higher than the end-of-season hospitalization rates for 2014-2015, a high severity, H3N2-predominant season," the CDC said in its FluView summary.

As has been typical all season, the highest rate of hospitalizations was among adults aged 65 years or older (370.6 per 100,000 population), followed by adults aged 50 to 64 (93.6 per 100,000 population) and children aged 0 to 4 years (62.5 per 100,000 population).

The vast majority of hospitalizations (80.2%) were associated with influenza A virus. H3N2 was found in 85.7% of hospitalized influenza A cases, with H1N1 accounting for 14.3%.

But nationally, influenza B is on the rise, as is typical in the latter part of the season. Of all specimens testing positive for flu last week, 3,090 (49.9%) were influenza A viruses and 3,103 (50.1%) were influenza B viruses. Of the influenza A viruses that were subtyped, 70.4% were H3N2 viruses and 29.6% were H1N1.

"The majority of people with influenza so far this season have been infected with the H3N2 influenza virus," Shuchat said during her testimony yesterday. "It is still too early to assess the full burden of influenza disease for this year, but estimates from recent seasons where H3N2 was predominant, like the 2012-13 and 2014-15 seasons, provide an indication of what to anticipate for this season.

"CDC estimated that during seasons like those, influenza accounted for as many as 35.6 million illnesses, 16.6 million medically attended visits, 710,000 hospitalizations, and 56,000 deaths."

See also:

Mar 8 CDC FluView

Mar 9 CDC FluView summary

Mar 8 Schuchat testimony

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