Influenza A outbreak in a nursing home: the value of early diagnosis and the use of amantadine hydrochloride.

An outbreak of respiratory tract infection due to influenza A virus occurred in February 1994 among residents of an 88-bed long term care facility in Toronto, Ontario. Eighteen confirmed and four probable cases of influenza were identified for an attack rate of 25%. Seventy-one per cent of the residents were known to have been immunized with trivalent influenza vaccine, but vaccine efficacy was determined to be only 32%. Influenza A viral antigen was rapidly detected by immunofluorescence microscopy of nasopharyngeal swabs, and amantadine hydrochloride was started in 76 residents within 48 h of recognizing the outbreak. Only two of the residents (3%) at risk became ill more than 48 h after amantadine was started. Amantadine was discontinued in four residents (5%) because of adverse effects (agitation, confusion, unsteady gait). In conclusion, rapid identification of the outbreak and prompt administration of amantadine hydrochloride prophylactically appears to be safe and effective in preventing further spread of influenza A.