Update: Influenza Activity -- United States, 1991-92 Season

From October 23, 1991, through January 18, 1992, 46 state health departments reported regional or widespread influenza activity for 1 or more weeks (Figure 1). For the week ending January 4, 34 states reported regional or widespread activity, the most during any single week this season. From late October through mid-December, influenza outbreaks reported from 11 states involved primarily school children (1). Reports of outbreaks among adults began in mid-November and continued through January and involved persons in a variety of settings (e.g., a business in California; a hospital in Ohio; a county jail in Tennessee; and nursing homes in Maryland, Missouri, New York, Ohio, Utah, and Wisconsin).

Based on CDC's 121-city mortality reporting system, for the week ending January 18, 7.9% of reported deaths were associated with pneumonia and influenza -- substantially exceeding the baseline level of 6.4% for that week. In addition, the week ending January 18 was the fourth consecutive week this index of influenza activity substantially exceeded baseline levels (Figure 2).

World Health Organization (WHO) collaborating laboratories in the United States identified more than 99% of all isolates as influenza A; 82% of all subtyped isolates were influenza A(H3N2). The proportion of influenza A(H1N1) viruses among subtyped isolates varied both regionally and temporally. Overall, from November 9 through January 18, A(H1N1) isolates increased from 8% to 18% of all influenza A isolates subtyped. Regionally, the proportion of subtyped influenza A isolates that were influenza A(H1N1) varied widely, ranging from 0 and 3%, respectively, in the East South Central and East North Central regions to 37% and 58%, respectively, in the Middle Atlantic and South Atlantic regions.

Of the influenza A(H3N2) viruses characterized at the WHO Collaborating Center for Influenza at CDC, 98% were antigenically closely related to the A/Beijing/353/89(H3N2) vaccine strain. Of the 33 strains of influenza A(H1N1) antigenically characterized at CDC, 23 (70%) were similar to the A/Taiwan/1/86 vaccine strain; however, 10 (30%) were similar to an antigenic variant represented by A/Texas/36/91 that is less inhibited by either antiserum to A/Taiwan/1/86 or antiserum to other related influenza A(H1N1) viruses (e.g., A/Sichuan/4/88) (Table 1). Influenza A/Texas/36/91-like viruses were isolated in the New England, Middle Atlantic, West North Central, West South Central, Mountain, and Pacific regions.

Nationally, the percentage of patient visits to sentinel physicians attributed to influenza-like illness increased steadily from a baseline of less than or equal to 2% of all office visits before October 19 to a sustained peak of 8%-10% from December 7 through January 4, then declined to 6%-7% of visits during the next 2 weeks.

Through January 18, all outbreaks among adults were reported from regions where 63%-100% of subtyped influenza isolates were A(H3N2) and were associated with either influenza A(H3N2) or influenza A (not subtyped). The only confirmed outbreak of influenza A(H1N1) was reported at a school in the South Atlantic region, where 58% of all subtyped influenza A viruses were A(H1N1). Reported by: Epidemiology Activity, Biometrics Activity, Office of the Director, and WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Div of Surveillance and Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: For the 1991-92 influenza season, sustained reporting of regional and widespread influenza activity began earlier than any time during the previous five influenza seasons (regionally, 2-7 weeks earlier, and widespread, 5-9 weeks earlier) (CDC, unpublished data, 1986-1992). Except during the 1990-91 season, when influenza B viruses dominated, an excess proportion of deaths attributed to pneumonia and influenza occurred during the five previous seasons. For the 1991-92 season, excess influenza-associated mortality was first evident during the last week of December, 1-8 weeks earlier than for the previous five seasons.

The Immunization Practices Advisory Committee recommends that vaccine continue to be offered to both children and adults at high risk for complications of influenza after influenza virus activity is documented in the community (2). The predominance of influenza A among circulating viruses indicates that amantadine is a reasonable option for prophylaxis of these vaccinees during the 2-week, postvaccination period while immunity develops.

Summaries of the rapidly changing national influenza surveillance data are updated weekly throughout the influenza season and are available by computer to subscribers to the Public Health Network and to the public through the CDC Voice Information System, telephone (404) 332-4555.

References

1. CDC. Update: influenza activity -- United States, 1991-92. MMWR 1991;40:809-10.

2. ACIP. Prevention and control of influenza: recommendation of the Immunization Practices Advisory Committee (ACIP). MMWR 1991;40(no. RR-6).

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