CDC: Flu rises sharply, now widespread in 36 states

Flu activity rose sharply last week, with 36 states now reporting widespread circulation and other markers such as clinic visits for flu and positive respiratory specimens also showing steep increases, the US Centers for Disease Control and Prevention (CDC) said today in its latest update.

The new report underscores a warning that came from the CDC 2 days ago about steadily climbing flu dominated by H3N2, a subtype known for its heavy impact, especially on seniors and young children. Between that and known gaps in H3N2 protection in seasonal flu vaccines, the CDC also reminded providers that antiviral drugs are an important second line of defense in treating flu.

Closer look at rising US flu markers

In its report today, the CDC said since its last report, the number of states reporting widespread flu activity—encompassing a broad swath of the country—jumped from 23 to 36, and the proportion of respiratory samples testing positive for flu at clinical labs rose from 14% to 22.4%.

Nationally, the percentage of clinic visits for flu—which climbed from 3.5% to 5%—has been at or above the national baseline for 5 weeks so far this season, the CDC said, adding that in recent seasons, flu has been at those levels for an average of 13 weeks.

In its chart comparing clinic visits this year to the past five seasons, the pattern seems to be closely following the moderate-to-severe 2014-2015 season, which was also dominated by H3N2.

High flulike illness activity, another marker of clinic visits for flu, was reported in 21 states, with regional activity reported by 13 states and Puerto Rico.

Flu hospitalizations are rising steadily, and last week were at an overall rate of 8.7 hospitalizations per 100,000 population. The levels were much higher in seniors, at 35.8 per 100,000 population.

Three more pediatric flu deaths were reported, raising the total to 12 this season. One was from an unsubtyped influenza A virus, while the other two were linked to influenza B.

Overall deaths from pneumonia and flu, an indicator that typically lags other flu markers, are below the seasonal baseline.

In tests last week at public health laboratories, 89% of positive flu specimens were influenza A and 11% were influenza B. Of the subtyped A viruses, 88.1% were H3N2. Analysis of a subset of H3N2 viruses shows extensive genetic diversity with many clades and subclades circulating, most of them similar to a cell-propagated reference virus that is a component of Northern Hemisphere flu vaccines.

Flu in Canada and Europe

Canada, which has also been experiencing early flu activity, reported a notable increase in flu outbreaks and hospitalizations in the most recent update from the Public Health Agency of Canada (PHAC), covering the week ending Dec 16.

Though H3N2 is the dominant strain, Canada is seeing a substantially greater number of influenza B illnesses for this time of year compared to past seasons. Levels of hospitalizations and deaths are highest in seniors, and since early November, surveillance has reported above-average hospitalizations for flu among children.

In Europe, where flu activity began more slowly, disease levels are on the rise in western, northern, and southern Europe, according to a joint report covering the week ending Dec 24 from the European Center for Disease Prevention and Control (ECDC) and the World Health Organization (WHO) regional office for Europe. Both influenza A and influenza B viruses are circulating, with mixed patterns detected across the region, including higher proportions of influenza B than influenza A and more 2009 H1N1 than H3N2. An analysis of a small number of H3N2 viruses suggests that the circulating viruses belong to the vaccine virus clade.

At sentinel healthcare sites, the percentage of respiratory specimens that tested positive for flu rose from 28% to 32% compared to the previous week.

See also:

Dec 29 CDC FluView update

Dec 29 CDC situation update

Dec 27 CIDRAP News story "Jump in H3N2 flu prompts CDC warning, antiviral reminder"

Dec 22 PHAC update

Dec 29 Flu News Europe report

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