Zanamivir Use During Transmission of Amantadine-Resistant Influenza A in a Nursing Home

OBJECTIVE: To describe the use of zanamivir during an influenza A outbreak. POPULATION: Residents of a 176-bed long-term-care facility for the elderly in Newmarket, Ontario, Canada, 90% of whom received influenza vaccine in the fall of 1998. OUTBREAK: When respiratory illness due to influenza A was confirmed, infection control measures and amantadine prophylaxis were initiated. Despite these measures, transmission of influenza A continued. INTERVENTION: Zanamivir inhalations, 10 mg daily for prophylaxis and 10 mg twice daily for treatment of influenza. RESULTS: There were 13 definite and 66 probable outbreak-associated cases of influenza A. Twelve (15%) cases developed pneumonia, 7 (9%) were hospitalized, and 2 (2.6%) died. All 12 culture-positive cases yielded influenza A/Sydney/H3N2/05/97-like virus, a 1998/99 vaccine component. The three isolates obtained prior to the initiation of amantadine were amantadine-susceptible; all nine obtained after prophylaxis was instituted were amantadine-resistant. One hundred twenty-nine (92%) of 140 residents who were offered zanamivir accepted it and were able to attempt inhalations. Of these 129, 78% (100) had no difficulty in complying with inhalations. Difficulty with inhalations was associated with decreased functional and mental status. Fifteen (58%) of 26 residents fully dependent in activities of daily living had difficulty compared to 14 (14%) of 100 others (P<.001). Twenty-two (45%) of 49 residents not oriented to person, place, or time had difficulty compared to 7 (10%) of 77 others (P<.001). In the 2 weeks after zanamivir prophylaxis, only 2 new cases of respiratory illness occurred, neither confirmed as influenza. No side effects were identified in 128 zanamivir-treated residents. CONCLUSION: A minority of nursing home residents have difficulty following instructions for zanamivir inhalations. Zanamivir was well tolerated, and its use was temporally associated with termination of an outbreak that amantadine had failed to control.