Update: Influenza Activity -- United States, 1992-93 Season

From September 27, 1992, through February 13, 1993, the World Health Organization collaborating laboratories in the United States reported 1042 influenza virus isolates. For the weeks ending December 5, 1992, through February 6, 1993, the ratio of specimens positive for influenza virus to total specimens submitted for respiratory virus testing increased from 0.02 to 0.2.

Weekly reports by state and territorial epidemiologists also indicated increasing levels of influenza-like illness (ILI) from December through mid-February. For the week ending February 13, 13 states reported sporadic ILI activity, 18 reported regional activity, and eight reported widespread activity.* Most laboratory-confirmed outbreaks of influenza occurred among school-aged populations, and most were associated with influenza type B.

From September 27, 1992 (the beginning of the 1992-93 influenza surveillance period), through February 13, 1993, 965 (93%) of influenza virus isolates reported to CDC were influenza type B. Although influenza type A viruses had been circulating at relatively low levels, the number and proportion of influenza type A viruses increased from January to mid-February. From September 27, 1992, through January 16, 1993, 10 (2%) of the 554 influenza viruses reported were influenza type A compared with 67 (14%) of the 488 viruses reported for January 17 through February 13. Influenza type B and type A viruses have been detected in 43 and 29 states and the District of Columbia, respectively (Figure 1, page 137). Of the 77 influenza A viruses detected, 13 were subtyped as A(H1N1) and 30 as A(H3N2); 34 have not been subtyped. Influenza type B viruses isolated in the United States this season have been antigenically similar to the B/Panama/45/90-like virus included in the 1992-93 influenza vaccine.

The proportion of deaths associated with pneumonia and influenza to total deaths reported through CDC's 121-city mortality reporting system during the 1992-93 influenza season have not exceeded baseline levels.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. WHO collaborating laboratories. Sentinel Physicians Influenza Surveillance System of the American Academy of Family Physicians. Influenza Br, and Epidemiology Activity, Office of the Director, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Influenza surveillance data for the 1992-93 season have shown patterns characteristic of seasons during which influenza type B has predominated. These patterns include the occurrence of outbreaks primarily among school-aged populations, limited outbreak activity among older adults, and no apparent excess in influenza-associated mortality. Among older adults, outbreaks of ILI with high attack rates over a short period of time are more often associated with influenza type A(H3N2) than with influenza type A(H1N1) or type B.

Increased circulation of influenza type A(H3N2) virus could elevate the risk of outbreaks in nursing homes and facilities housing elderly persons. Such facilities should develop contingency plans for the rapid administration of amantadine in the event of suspected or confirmed influenza type A outbreaks (1). Specimens for virus isolation or antigen detection should be obtained from ill residents before amantadine is administered. Facilities that do not have information regarding resources available for laboratory diagnosis of influenza should contact their local or state health department.

Influenza surveillance data are updated weekly throughout the influenza season, and summaries are available by computer to subscribers of the Public Health Network and to health-care providers and the public through the CDC Voice Information System, telephone (404) 332-4555.

Reference

1. ACIP. Prevention and control of influenza: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1992;41(no. RR-9).

• Levels of activity are: 1) sporadic -- sporadically occurring ILI or culture-confirmed influenza, with no outbreaks detected; 2) regional -- outbreaks of ILI or culture-confirmed influenza in counties having a combined population of less than 50% of the state's total population; 3) widespread -- outbreaks of ILI or culture-confirmed influenza in counties having a combined population of 50% or more of the state's total population.

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