Decline in US flu activity accelerates

Apr 10, 2009 (CIDRAP News) – The spring decline of US influenza activity picked up speed last week, when only four states still reported widespread flu activity, down from 13 a week earlier, the Centers for Disease Control and Prevention (CDC) reported today.

Eighteen states reported regional flu activity for the week that ended Apr 4, while another 20 had only local activity, the agency said. The rest of the states had only sporadic cases or didn't report. The states still dealing with widespread flu were Alabama, New York, Virginia, and Washington.

Despite some problems with antiviral resistance, this season appears to have been a fairly mild one, at least in comparison with last year, when two of the three flu strains in the vaccine didn't match up well with the viruses making people sick.

Medical visits to the CDC's network of sentinel providers for treatment of influenza-like illness (ILI) this season have run well below last season. A chart in the CDC report shows that the proportion of visits for ILI this season peaked in February at about 3.5% of all visits, whereas at last year's peak, also in February, ILI accounted for about 6% of all medical visits. Last week the proportion of visits ascribed to ILI was 1.6%, well below the epidemic threshold of 2.4%.

Two flu-related deaths in children were reported last week, one each in Illinois and Virginia, the CDC reported. That brings the seasonal total to 45 deaths, which is well below the overall toll of 88 for the 2007-08 season and 78 for 2006-07.

Of the 45 children who died, 27 were tested for secondary bacterial infections, and 12 tested positive. Eight of these had Staphylococcus aureus infections, three of which involved methicillin-resistant (MRSA) strains.

In another sign of a fairly mild season, flu-related hospitalization rates for children under age 4 this season have been well below those of last season, the CDC report shows. Through Mar 21, data from the New Vaccine Surveillance Network show the rate for this season was 1.46 hospitalizations per 10,000 children.

Flu-related hospitalization rates for adults this season also were well below last season's level, according to data from the CDC's Emerging Infections Program (EIP). Rates for adults this year were fairly close to those recorded in 2006-07, a chart based on EIP data shows.

As for overall flu-related deaths, 7.4% of deaths reported through the CDC's 122-city mortality surveillance system last week were attributed to pneumonia and influenza, which is below the epidemic threshold of 7.8%, the CDC said. A chart shows that the percentage of deaths attributed to pneumonia and flu never surpassed the epidemic threshold this season, whereas it hovered above that level for several weeks last season.

Continuing a trend of the past several weeks, influenza B viruses were the most common type identified last week, though type A viruses have been predominant in the season overall. Of 348 viruses that were typed, 203 (58.3%) were type B. For the whole season, 67.9% of isolates have been type A and 32.1% type B.

Most of the type B viruses tested this year have not been a good match for the B strain in this year's vaccine. The latest numbers show that of 263 type B viruses that have been antigenically characterized, 50 belonged to the Yamagata lineage used in the vaccine, while 213 belonged to the Victoria lineage and were not related to the vaccine strain.

However, all influenza A isolates that the CDC has studied have been well matched to the strains in this year's vaccine. These include 594 A/H1N1 viruses and 88 A/H3N2 isolates, the CDC reports.

Among influenza A viruses, the H1N1 subtype has predominated over H3N2 this season. That may be a factor in the relatively mild season, as experts say that seasons tend to be more severe when H3N2 viruses dominate.

Today's report shows a continuation of the season-long pattern of oseltamivir (Tamiflu) resistance in H1N1 viruses. It says 694 of 699 (99.3%) tested H1N1 isolates showed resistance to the drug, while all tested H3N2 and B viruses have been sensitive to oseltamivir and the other neuraminidase inhibitor, zanamivir (Relenza). The resistance findings prompted the CDC to revise its recommendations for antiviral treatment in December.

Nearly all H1N1 isolates tested this season have been sensitive to the older antiviral drugs, amantadine and rimantadine. However, all tested H3N2 viruses have shown resistance to those drugs.

See also:

CDC flu update
http://www.cdc.gov/flu/weekly/

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