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

From September 27, 1992, through January 19, 1993, 344 influenza virus isolates were reported in 29 states (Figure 1). The number of reported isolates began to increase in early December and continued to increase during the first 2 weeks of January.

For the week ending January 16, five states (Alaska, Arkansas, California, Missouri, and Texas) reported regional activity, and three (New Mexico, New York, and Washington) reported widespread activity.* The number of states reporting sporadic influenza-like illness (ILI) increased from five states for the week ending October 3 to 23 states for the week ending January 16. Based on CDC's 121-city mortality reporting system, deaths associated with pneumonia and influenza have not exceeded baseline levels.

World Health Organization collaborating laboratories in the United States identified 98% of all isolates as influenza type B. Although influenza type A has circulated at low levels, both influenza type A(H1N1) and type A(H3N2) viruses have been isolated.

School outbreaks of ILI were reported from Arizona, Arkansas, Missouri, and Washington. All of these states reported isolation of influenza type B viruses from various sources. In Washington, influenza type B was isolated from specimens obtained from ill students attending schools with outbreaks.

The first outbreak this season of influenza in a nursing home was reported from Washington. Nineteen (20%) of 97 residents became ill during December 28, 1992-January 5, 1993; influenza type B was isolated from three of six specimens obtained from ill residents. As with virtually all influenza type B viruses isolated in the United States this season, these isolates were antigenically similar to the B/Panama/45/90-like virus included in the 1992-93 influenza vaccine. Ninety-four (97%) of the residents had received influenza vaccine in October 1992. Two residents, both with severe underlying diseases, died within 2 weeks of developing ILI. Most residents, however, had relatively mild illnesses compared with those observed in the same facility when an outbreak caused by influenza A(H3N2) occurred during the winter of 1991-92 (CDC, unpublished data, 1992).

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. R Atwood, MD, D Hursh, Yakima County Health District; S LaCroix, MS, P Shoemaker, JM Kobayshi, MD, State Epidemiologist, Washington Dept of Health. J Marquez, MPH, L Sands, DO, State Epidemiologist, Arizona Dept of Health Svcs. D Berry, MS, Arkansas Dept of Health. I Donelin, HD Donnell, Jr, MD, State Epidemiologist, Missouri Dept of Health. WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza. 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: Although most influenza viruses detected this season have been influenza type B, health-care providers should continue to test specimens from persons with ILI throughout the influenza season. The proportions of different influenza virus types or subtypes can change substantially during the season.

It is particularly important to differentiate between influenza A or B as the cause of outbreaks of ILI in institutions housing high-risk persons because amantadine can be used to treat ill persons and prevent further spread of infection during outbreaks caused by influenza type A (1). Rapid antigen-detection testing can be performed at the site of an outbreak, and, if present, influenza type A can be identified within 15 minutes from a nasopharyngeal swab specimen (2). If any person in an outbreak setting tests positive for influenza type A, it should be assumed that influenza type A is the cause of the outbreak. Results of rapid antigen-detection tests should be confirmed by virus isolation. However, when a decision has been made to use amantadine, initiation of amantadine prophylaxis or treatment should not be delayed pending confirmation of virus type. Guidelines for the use of amantadine to control influenza type A in chronic-care facilities have been published (1).

With the increase in influenza activity, it is important that children and teenagers avoid the use of aspirin and aspirin-containing products because of the increased risk of developing Reye syndrome when aspirin is taken during an ILI (3).

References

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

2. Waner JI, Todd SJ, Shalaby H, Murphy P, Wall LV. Comparison of Directogen FLU-A with viral isolation and direct immunoflorescence for the rapid detection and identification of influenza A viruses. J Clin Microbiol 1990;29:479-82.

3. Hurwitz ES, Barrett MJ, Bregman D, et al. Public Health Service study of Reye's Syndrome and medications: report of the main study. JAMA 1987;257:1905-11.

*Levels of activity are: 1) sporadic -- sporadically occurring influenza-like illness (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; and 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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