Flu activity continues its slow retreat

Feb 22, 2013 (CIDRAP News) – Some of the key markers that US officials use to track flu continued to tumble last week, though activity was still elevated in most parts of the country, according to the Centers for Disease Control and Prevention (CDC) today.

Nationally, the percentage of doctors visits for flulike illness fell from 3.2% to 2.8% last week, with all parts of the country still above regional baselines.

The overall percentage of respiratory samples that tested positive for influenza fell from 19.7% to 16.8% last week. H3N2 is still the dominant flu strain, as it has been all season, though just over 45% of viral detections were influenza B.

The CDC received reports of 14 more pediatric flu deaths last week, raising the season's total to 78 so far. Several of the deaths occurred earlier in the season. Six were linked to influenza B, three to H3N2, two to 2009 H1N1 (pH1N1), and two to unsubtyped influenza A viruses. A child who died during the week ending January 5 was co-infected with influenza A and influenza B. The two pH1N1 deaths were the first pediatric ones reported this season from the former pandemic virus.

Overall, the percentage of deaths from pneumonia and flu was still well above the epidemic threshold, but it fell last week from 9.1% to 8.6%.

One flu indicator that continued to rise was the rate of flu-related hospitalizations, from 32.1 to 34.2 per 100,000 population last week. Though hospitalizations are for the most part leveling off, serious illnesses in seniors appear to be pushing that indicator higher, the CDC said.

Seniors make up more than half of hospitalizations for flu. This age-group is among the most vulnerable to influenza complications and has a weaker response to flu vaccination. Yesterday in its midseason flu vaccine effectiveness estimate the CDC reported that the vaccine didn't have a significant impact on protecting seniors, a pattern seen in Canada and Europe, as well.

Twenty-two states reported widespread geographic spread of flu, down from 31 the week before.

Elsewhere in North America, flu activity in Canada was at expected levels for this time of year, and, as in the United States, the highest flu-related hospitalization rates were in people age 65 or older, according to an update yesterday from the Pan American Health Organization (PAHO).

H3N2 is still the most commonly detected flu virus in the United States, Canada, and Mexico, with levels of respiratory syncytial virus (RSV) rising in the United States and Canada to levels close to flu, PAHO reported.

Meanwhile, European health officials today reported decreases in some of the region's flu markers, though substantial activity is still under way. The European Centre for Disease Prevention and Control (ECDC) said the percentage of respiratory specimens that tested positive for flu last week was still high at 52%, but it decreased for the second consecutive week.

Four countries reported high-intensity flu activity: Belgium, Germany, Luxembourg, and, for the first time this season, Finland. Most countries reported stable flu activity. The ECDC said despite ongoing substantial activity, it appears that more countries are passing or have passed their flu peaks.

Testing shows an even mix between influenza A and influenza B viruses in Europe, with an increasing proportion of pH1N1 viruses since the end of 2012.

See also:

Feb 22 CDC influenza update

Feb 22 CDC flu situation update

Feb 22 ECDC influenza update

Feb 21 PAHO update

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