Update: Influenza activity—United States, 2001-02 season

Morbidity and Mortality Weekly Report, Dec 6, 2002

This report summarizes influenza activity in the United States during the weeks ending October 5-November 23, 2002 *. Influenza activity was low during this period but is expected to increase during the coming weeks. Vaccine supplies are plentiful this year and vaccination should continue throughout December and the remainder of the influenza season, particularly among persons at high risk for complications from influenza.

As of November 23, the World Health Organization (WHO) and the National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 10,036 respiratory specimens for influenza viruses, of which 47 (0.5%) were positive. For the weeks ending October 5-November 23, the weekly percentages of respiratory specimens testing positive for influenza viruses ranged From 0 to 2.3%. During the 1999-2000, 2000-01, and 2001-02 influenza seasons, respectively, the peak percentages of specimens testing positive for influenza ranged from 23% to 31% (2 CDC, unpublished data, 2002).

Of the 47 positive isolates, 20 were influenza A and 27 were influenza B viruses. Six of the 20 influenza A isolates were subtyped as influenza A(H1) [+] viruses. The A(H1) isolates were reported from North Carolina and Florida. One influenza A(H3N2) virus was reported from Hawaii. Influenza A isolates not yet subtyped were reported from Louisiana, Nebraska, New York, South Carolina, and Virginia. Influenza B isolates were identified in Louisiana, New York, South Carolina, and Texas. In addition to the isolates reported through WHO and NREVSS laboratories, California reported an influenza B isolate.

CDC has characterized antigenically seven influenza viruses submitted by U.S. laboratories since September 29: five influenza B, one influenza A(H3N2), and one influenza A(H1N2). The influenza B viruses, the A(H3N2) virus, and the hemagglutinin protein of the A(H1N2) virus were similar antigenically to the corresponding vaccine strains B/Hong Kong/330/01, A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively.

Since 1977, two influenza A virus subtypes, A(H1N1) and A(H3N2), have co-circulated in humans. During the 2001-02 season, influenza A viruses with the H1 hemagglutinin and the N2 neuraminidase protein were identified. These A(H1N2) viruses appear to have resulted from the reassortment of genes between the currently circulating A(H1N1) and A(H3N2) viruses (3). Influenza A(H1N2) viruses have been identified in several countries, including the United States. Four of the six H1 viruses reported in the United States since September 29 have been subtyped; one virus from North Carolina was an A(H1N2) virus, and three from Florida were A(H1N1) viruses. The 2002-03 vaccine contains A(H1N1) and A(H3N2) viruses and is expected to provide protection against influenza A(H1N2) viruses (4).

During the weeks ending October 5-November 23, the weekly percentages of patient visits for influenza-like illness (ILI) [section] to approximately 750 sentinel providers ranged from 1.0% to 1.3%. For the week ending November 23, the percentage of patient visits for ILI was 1.3%, which is less than the national baseline of 1.9% [paragraph]. During the 1999-2000, 2000-01, and 2001-02 seasons, the peak percentages of patient visits for ILI ranged from 3.2% to 5.6% (2; CDC, unpublished data, 2002).

During the week ending November 23, influenza activity ** was reported by state and territorial epidemiologists as regional in Louisiana and Texas and as sporadic in 18 states (Colorado, Florida, Georgia, Hawaii, Indiana, Kansas, Kentucky, Maine, Michigan, Missouri, Nebraska, Nevada, New Mexico, Pennsylvania, Tennessee, Utah, West Virginia, and Wyoming) and the District of Columbia; 28 states and New York City reported no influenza activity. Two states did not report. Since the week ending October 5, Florida, Louisiana, and Texas have reported regional activity for at least 1 week.

During October, Louisiana reported an influenza outbreak in schools in Lafourche Parish. The outbreak peaked during the week ending October 26 and affected primarily persons aged 5-24 years. Since October 5, a total of 102 cases of influenza B have been diagnosed by rapid test or viral isolation in Lafourche Parish.

During October 1-17, a total of 30 patients with ILI were admitted to the infirmary of a north Florida prison, which employs 300 staff and houses 1,000 prisoners. Influenza A(H1) was confirmed by culture in three cases. The estimated number of patients in the outbreak was 100-150, including two staff.

For the week ending November 16, Texas reported an outbreak of respiratory illness affecting mostly elementary school children. On November 19, a total of 12 specimens collected from children aged 6-11 years with ILI attending an east Texas school in Smith County were submitted to the Texas Department of Health Virology Laboratory for culture confirmation. Ten of these cultures were positive for influenza B, and two are pending. During the outbreak, absentee levels for local schools in Smith County approached 25%.