Outbreak of Influenza A in a Nursing Home -- New York, December 1991 - January 1992

During December 1991 and January 1992, an outbreak of influenza A occurred among residents of a nursing home in New York. From mid-October through mid-November 1991, influenza vaccine was administered to 295 (88%) of the 337 residents of the nursing home. The residents shared common dining and recreational areas; the mean age of residents was 83 years (range: 51-103 years); 256 (76%) were female. The prevalence of underlying illness in vaccinated and unvaccinated patients was not determined.

From December 9 through January 10, 65 (19%) of the residents had onset of influenza-like illness (ILI) * (Figure 1). Among those with ILI, 34 (52%) developed pneumonia, 19 (29%) required hospitalization, and two died. In addition to fever, symptoms included cough (72%), congestion (57%), pharyngitis (38%), and rhinitis (8%). Isolates from throat-swab specimens obtained from two patients were identified as influenza A(H3N2) by the New York State Wadsworth Center for Laboratories and Research.

ILI occurred among 52 (18%) vaccinated residents and 13 (31%) unvaccinated residents (relative risk=1.8; 95% confidence interval (CI)=1.1-2.9). Pneumonia following ILI occurred among 27 (9%) vaccinated residents and seven (17%) unvaccinated residents. The calculated vaccine efficacy for preventing ILI and pneumonia was 43% (95% CI=5%-66%) and 45% (95% CI=|m-18%-74%), respectively.

On January 6, the New York State Department of Health was notified of the outbreak and recommended amantadine therapy for patients with ILI. However, medical staff administered amantadine only to a limited number of residents.

Questionnaires were distributed to nursing home employees with patient contact to determine the vaccination coverage and incidence of ILI among these staff. Of 449 employees, 339 (76%) completed questionnaires. Thirty-three (10%) employees had been vaccinated in the fall before or during the outbreak. From November 1 through January 9, 65 (19%) had ILI; the calculated vaccine efficacy for preventing ILI in nursing home employees was 86% (95% CI=34%-99%).

Reported by: C Page, N Crispo, Rensselaer County Health Dept; GS Birkhead, MD, P Drabkin, MPH, S Kondracki, DL Morse, MD, State Epidemiologist, New York State Dept of Health. Office of the Director, Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The 1991-92 influenza season has been characterized by widespread activity and associated with substantial mortality in the United States (1). Influenza A(H3N2) has been the predominant cause of illness during the 1991-92 influenza season, and outbreaks associated with this virus have been reported in a variety of settings (1). The outbreak described in this report underscores the potentially severe impact of influenza A among residents of nursing homes and other high-risk groups in institutional settings.

Estimates of influenza vaccine effectiveness may vary from year to year and among different groups during a given year. Influenza vaccine efficacy is influenced by various factors, including host immune response to the vaccine and the degree of antigenic similarity between the circulating influenza virus strain(s) and those included in the vaccine. The calculated vaccine effectiveness is also influenced by various factors. These include

1. the extent to which persons are exposed to the circulating virus, 2) the presence of immunity to the virus that is independent of vaccination (e.g., through immunity induced by a previous influenza infection), and 3) the reduced susceptibility to infection among persons receiving amantadine prophylaxis during the time of exposure to the virus. In this outbreak, the calculated vaccine effectiveness against ILI and pneumonia is within the range of vaccine effectiveness estimates found during other nursing home outbreaks caused by influenza A(H3N2) viruses that are similar to the vaccine strain (2).

Because the effectiveness of influenza vaccine may vary, and because many persons at increased risk for serious complications of influenza may not be vaccinated before exposure during an outbreak, amantadine chemoprophylaxis has been recommended as an additional preventive measure during influenza A outbreaks (3,4). Amantadine prophylaxis may be a particularly helpful adjunct for controlling influenza in groups of persons in nursing homes and other institutional settings (3,4).

Other influenza outbreaks in health-care settings have also documented a low level of vaccination among employees that may have contributed to the outbreaks (5,6). Influenza vaccine is recommended for all health-care workers and others in close contact with high-risk persons to reduce their exposure to influenza from care providers (4).

References

1. CDC. Update: influenza activity -- United States, 1991-92 season. MMWR 1992;41:63-5.

2. Patriarca PA, Arden NH, Koplan JP, Goodman RA. Prevention and control of type A influenza infections in nursing homes: benefits and costs of four approaches using vaccination and amantadine. Ann Intern Med 1987;107:732-40.

3. CDC. Control of influenza A outbreaks in nursing homes: amantadine as an adjunct to vaccine -- Washington, 1989-90. MMWR 1991;40:841-4.

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

5. Pachuki CT, Pappas SA, Fuller GF, Krause SL, Lentino JR, Schaaf DM. Influenza A among hospital personnel and patients: implications for recognition, prevention, and control. Arch Intern Med 1989;149:77-80.

6. Coles FB, Balzano GJ, Morse DL. An outbreak of influenza A(H3N2) in a well immunized nursing home population. J Am Geriatr Soc (in press).

   
   

• Illness characterized by fever greater than or equal to 100 F (greater than or equal to 38 C) with cough or sore throat.

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