US flu picking up pace; global activity mixed

Jan 28, 2011 (CIDRAP News) – Influenza activity in the United States last week increased in intensity and extent, sending more people to doctor's offices, the US Centers for Disease Control and Prevention (CDC) said today.

Nearly all flu measures increased, and half of the states are reporting widespread activity, rising from 17 to 25 over the previous week. Six of the CDC's 10 regions saw doctor's visits for influenza-like illness (ILI) rise above their baselines, compared with 3 the week before. Nationally, the proportion of doctor's visits for ILI was 3.6%, which is above the 2.5% national baseline.

The only areas still reporting flu-related doctor visit levels at or below baseline are the far northeastern states and western states.

The latest findings appear to confirm the CDC's assessment earlier this month that the season hadn't reached its peak yet. In a summary of its weekly flu surveillance report today, the CDC said activity in the United States generally peaks in January or later.

The level of deaths from pneumonia and flu decreased slightly over the past week, and at 7.5% of all deaths is about what federal health officials expect at this point in the flu season. Three more pediatric deaths were reported, raising the season's total to 13. Two of the deaths were attributed to influenza A/H3N2, and the other was linked to influenza B.

About 30% of respiratory specimens tested positive for influenza, up from 25.9% the week before.

The dominant flu strain is still H3N2, with about 20% of illnesses caused by influenza B. Of the influenza A viruses subtyped, 36% were the 2009 H1N1 virus, about twice as high as the week before. The CDC said the circulating strains are still a good match with the viruses included in the seasonal flu vaccine.

Globally, the World Health Organization (WHO) reported today that flu activity is increasing in Europe, particularly in the west, but the United Kingdom—which a few weeks ago had reported levels higher than during the 2009 H1N1 pandemic—now is seeing decreasing levels. The agency also said flu transmission seems to be leveling off or decreasing in much of North America.

All three strains—2009 H1N1, H3N2, and B—are circulating across the globe, but the strains are well matched to those in the flu vaccine, the WHO said. Also, severe cases and deaths have been associated with all three strains, the report said.

In Canada, rates of ILI and hospital admissions for flu have decreased in the last 2 weeks, though one region in Ontario has reported increased rates. The country's ILI reporting is similar to influenza seasons before 2009. About 23% of specimens tested in Canada were positive for influenza, down slightly from recent weeks.

Mexico is seeing more influenza B than either Canada or the United States, where H3N2 predominates.

As for Europe, where 2009 H1N1 dominates, today's report says, "Many countries of Western Continental Europe are now reporting significant numbers of severe and fatal cases of influenza though intensive care units (ICU) have not yet been stressed to the degree [previously] seen in the UK."

In Denmark, the percentage of flu patients in ICUs rose from 4% 2 weeks ago to 9.5% in the past week. In France, where 290 flu patients have been treated in ICUs, 41% had no underlying condition, a higher percentage than seen in the United Kingdom. Norway, bucking Europe's trend, is experiencing a predominance of type B flu.

In the tropics, some South Asia countries, such as Singapore and Hong Kong, are seeing increasing flu activity, mainly owing to 2009 H1N1.

Worldwide, 78.6% of flu specimens have been influenza A and 21.4% influenza B. Of the influenza A specimens subtyped, 74.5% have been 2009 H1N1 and 25.4% have been H3N2.

Editorial Director Jim Wappes contributed to this report.

See also:

Jan 28 CDC flu update summary

Jan 28 CDC flu weekly flu surveillance update

Jan 28 WHO update

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