US flu ebbs but still causes considerable illness

Feb 8, 2013 (CIDRAP News) – US flu indicators continued their decline last week, though many states are still reporting brisk levels of flu activity, the Centers for Disease Control and Prevention (CDC) reported today.

Clinic visits for influenza-like illness (ILI) are still above baseline in all 10 of the CDC's regions, though the national marker fell to 3.6% last week, compared with 4.2% reported the week before. Based on clinic visit surveillance, 19 states and New York City reported high ILI activity, compared with 24 states and New York City the previous week. The intensity activity seen in western states appears to have eased a bit.

Nationally, the number of states reporting widespread geographic influenza spread dropped from 42 to 38 states last week. Nine states reported regional flu activity.

The percentage of respiratory swabs that tested positive for flu dropped last week to 23.3% from 25.5%.

Indicators for serious illness presented a mixed picture. The overall percentage of deaths from flu and pneumonia, while still markedly above the epidemic threshold, dropped from 9.4% to 9.0% last week.

The CDC received reports of 14 more pediatric deaths, though 11 of them occurred in earlier weeks. Seven of the deaths were from influenza B, four were from an undetermined influenza A type, and three were linked to H3N2. So far this season 59 pediatric flu deaths have been reported.

The rate of hospitalization for lab-confirmed flu, however, increased from 25.9 to 29.8 per 100,000 population last week. The most affected group is still people age 65 or older, which accounted for more than 50% of the reported cases.

The H3N2 virus still predominates, followed by influenza B. The CDC said predominant viruses have varied by state and region over the course of the flu season.

About 74% of specimens tested positive for influenza A during the week, compared with 24% for influenza B. Of the 1,100 influenza A viruses that were subtyped, 1,026 (93%) were H3N2, with the rest being 2009 H1N1 (pH1N1).

Elsewhere in North America, flu indicators in Canada have also decreased, though doctors' visits for ILI are still running above expected levels for this time of year, according to a Feb 5 update on respiratory virus activity from the Pan American Health Organization (PAHO).

Seven regions in Ontario, Quebec, and Newfoundland are still reporting widespread flu activity. H3N2 is still the dominant strain in Canada, and that country is still experiencing less influenza B compared with the United States.

Mexico reported that 32.7% of respiratory swabs tested positive for flu, with influenza A responsible for the majority of cases and H3N2 predominating among subtyped viruses.

In Europe, flu surveillance showed a mixed picture, according to an update today from the European Centre for Disease Prevention and Control (ECDC). Flu activity continued to rise across the continent, with signs that a few countries have passed their peaks, while the numbers showed a resurgence of flu activity in others.

Overall the percentage of respiratory samples in Europe that tested positive for flu last week was 55%, a level that has risen progressively over the past 3 weeks, according to the ECDC.

Belgium, Germany, Luxembourg, and Sweden reported high-intensity transmission. Increasing trends were reported by 22 countries and the United Kingdom. The countries that saw a resurgence of ILI after appearing to peak include Denmark, Greece, Ireland, and Luxembourg.

The proportions of circulating strains in Europe remained similar to previous weeks, at 51% influenza A and 49% influenza B, but the percentage of pH1N1 virus increased in recent weeks from 52% about 3 weeks ago to 64% last week.

See also:

Feb 8 CDC weekly influenza update

Feb 5 PAHO flu and other respiratory virus update

Feb 8 ECDC weekly influenza update

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