2003-04 flu season was tougher than last three

Apr 8, 2004 (CIDRAP News) – Looking back on the 2003-04 influenza season, federal health officials say it was rougher than the previous three seasons but not unusual for years when the predominant flu virus is A(H3N2).

Preliminary data show that "the current season was more severe than the previous three seasons but was within the range expected for a typical A(H3N2) season," the Centers for Disease Control and Prevention (CDC) says in the Apr 9 issue of Morbidity and Mortality Weekly Report.

The season came and went earlier than usual, with widespread flu activity first reported in some states in mid-October, according to the report. By mid-December (Dec 14-20), 45 states had widespread activity and four states had regional activity. But by Jan 24, only one state still had widespread activity.

The percentage of patient visits to 1,000 sentinel healthcare providers for flu-like illnesses exceeded the national baseline of 2.5% for 9 weeks, from Nov 15 through Jan 10, the CDC reports. The proportion peaked at 7.6% the week of Dec 21-27. The percentage of deaths attributed to pneumonia and flu stayed above the epidemic threshold of 8.2% for 9 weeks, from Dec 20 through Feb 14.

More than 99% of the 24,177 flu viruses identified in US laboratories during the season were type A. Almost all (99.9%) of the 6,875 type A viruses that were subtyped were identified as A(H3N2). The CDC antigenically characterized 833 A(H3N2) isolates and determined that 87.3% resembled A/Fujian/411/2002(H3N2), while 12.7% resembled A/Panama/2007/99/(H3N2).

Preliminary data show that 142 children (younger than 18) died of flu-associated illnesses during the season, according to the CDC. All but five deaths occurred before Jan 26. The agency says it is collecting further data on the cases, "and efforts are under way to track national pediatric influenza-associated deaths annually."

The report also notes that no human flu cases have been linked with any of the recent avian influenza outbreaks in the United States, though two human cases of influenza A(H7) have been associated with the current avian flu outbreak in British Columbia.

CDC. Update: influenza activity—United States, 2003-04 season. MMWR 2004;53(13):284-7 [Full text]

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