Editor's Note: The fourth paragraph was revised on Jan 2 to clarify the current situation regarding H3N2 dominance.
Dec 28, 2012 (CIDRAP News) – US influenza activity continued its ascent last week, judging from the high volumes of patients seeking care for the disease at doctors' offices, while flu indicators in Europe showed a clear spike, according to the latest surveillance reports.
Outpatient clinic visits for influenza-like illness (ILI) in the United States rose to 4.2% of all visits, well above the national baseline of 2.2%, according to the weekly report from the Centers for Disease Control and Prevention (CDC).
High levels of ILI activity were reported in 16 states and New York City, an increase from 12 states the previous week, the CDC said. The only area that wasn't over its baseline for this flu marker was the western region, which includes Arizona, California, Guam, Hawaii, and Nevada.
The US flu season started early this year and may end up being more severe than usual if early patterns hold, because the currently dominant flu strain is H3N2, which tends to cause more severe illnesses. In some hard-hit areas, such as parts of North Carolina, hospitals are limiting visitors to curb the spread of flu to high-risk patients.
Eight pediatric flu deaths were reported last week, pushing the season's total to 16. Three were linked to influenza B viruses, three were associated with H3N2, and the influenza A subtype wasn't determined for the other two. The percentage of deaths from pneumonia and flu remained below the epidemic threshold, but this indicator usually lags other flu markers.
The percentage of respiratory specimens testing positive for flu stayed high at 29.6%, up from 28.3% reported the week before, according to the CDC.
Labs that monitor the circulating flu strains report that H3N2 is still the dominant strain, followed by influenza B.
Few 2009 H1N1 viruses have been detected in the United States, but the virologic picture in Europe is different, with the virus accounting for close to a third of influenza A viruses that have been subtyped.
Canada, like its southern neighbor, is reporting predominantly H3N2 viruses with few 2009 H1N1 detections, according to the latest report (covering the week ending Dec 15) from the Public Health Agency of Canada (PHAC).
The flu hotspots in Canada so far are four regions of Ontario, which are reporting widespread flu activity. The PHAC said doctor visits for ILI rose from the previous week to a pace expected for this time of year.
In its update today the European Centre for Disease Prevention and Control (ECDC) said that even though surveillance activities have been limited over the holiday period, there are clear indications of rising flu activity, with the disease affecting more countries last week compared with the previous week.
The proportion of specimens that tested positive for flu spiked to 27.3%, compared with 17.3% the week before. Overall, 70% of the samples were influenza A, of which nearly 69% were H3N2. Influenza B made up 28% of the subtyped viruses.
The ECDC said testing suggests that the circulating viruses are well matched to the seasonal flu vaccine. Similar testing by the CDC also showed a close match, except for a few influenza B viruses that were from the Victoria lineage, which is not a component of this season's vaccine for the Northern Hemisphere.
Of 17 European countries that reported clinical data, France and Luxembourg reported medium-intensity flu activity, with the rest reporting low intensity. Three countries reported wide geographic spread: Denmark, the Netherlands, and the United Kingdom (England).
Increasing flu trends were reported by nine countries: the Czech Republic, Denmark, Estonia, France, Hungary, Latvia, Luxembourg, Slovakia , and the UK.
Dec 28 CDC weekly flu update
Dec 28 ECDC weekly influenza surveillance overview
Dec 21 PHAC weekly flu surveillance report