US flu season continues to hit elderly hard

Elderly woman in hospital
Elderly woman in hospital

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Although many markers of flu activity continue to decline slightly, hospitalizations for influenza keep climbing, with the rate again setting a record in seniors, the US Centers for Disease Control and Prevention (CDC) reported today in its weekly update.

Flu activity has been elevated for 13 consecutive weeks as of last week, the CDC said. Over the past 13 flu seasons, that has been the average length of elevated activity, so this season is promising to be an extended one.

The CDC also reported 6 new flu-related deaths in children, for a season total of 86 such deaths.

Indicators of declining activity

The proportion of people seeking healthcare for influenza-like illness (ILI) nationwide was at 3.2% last week, down from 3.7% the week before but still well above the baseline of 2.0%. Puerto Rico and 12 states reported high ILI activity, down from 15 states the week before. All 10 US regions, though, reported ILI activity at or above regional baselines.

Widespread flu activity was reported in 30 states and Puerto Rico, down a bit from 32 the week before.

The percentage of respiratory specimens testing positive for influenza decreased from 14.9% to 13.0% last week, according to the CDC. For the most recent 3 weeks, the regional percentages ranged from 8.8% to 24.6%.

Signs of severe disease

In contrast, the estimated cumulative rate of hospitalization caused by lab-confirmed flu increased from 44.1 hospitalizations per 100,000 to 48.6. During the same week of the 2012-13 season, which was similarly dominated by the H3N2 virus, the rate was 35.0 per 100,000 population.

The rate among the elderly, however, was a dramatic 242.2 per 100,000 population, up from 217.3 the week before. This is the highest rate among those 65 and older that the CDC has recorded since it began tracking the statistic in 2005. The previous high was 183.2 per 100,000, the rate for the entire 2012-13 season.

The six newly reported pediatric flu deaths occurred from mid-December to early February. Four were associated with H3N2, and two were linked to an influenza A virus that was not subtyped.

In yet another measure of severity, 8.4% of all deaths recorded via the CDC's 122-city reporting system were attributed to pneumonia and flu, which was above the epidemic threshold of 7.2% and up from 8.1% during the previous week.

Rising flu B, continued H3N2 mismatch

Influenza B reached its highest proportion among viruses tested thus far this season, which often happens late in the season. Of viruses tested, 77% were influenza A, and 23% were type B.

More than two thirds of the type B viruses subtyped—or 107 of 154—are of the Yamagata lineage, which is the "B" strain included in the trivalent (three-strain) vaccine. Quadrivalent vaccines contain both the Yamagata and Victoria subtypes.

Fully 512 of 518 influenza A viruses subtyped, or 99%, were H3N2, with the other 6 being 2009 H1N1.

From week to week more than two thirds of circulating H3N2 viruses have been mismatched with the H3N2 strain in the vaccine. That trend did not waver last week, with 69% of H3N2 viruses characterized as mismatched.

See also:

Feb 20 CDC FluView update

Feb 20 CDC FluView summary

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