Duration of antiviral prophylaxis during nursing home outbreaks of influenza A - A comparison of 2 protocols

Background: We performed a randomized trial of 2 protocols guiding the duration of antiviral chemoprophylaxis during outbreaks of influenza A in a rural, 700-bed nursing home for veterans and their spouses with 14 nursing units in 4 buildings. Methods: Half of all residents volunteered to participate. Nursing units were randomized, and the effectiveness of short-term (minimum, 14 days and 7 days without the onset of a case in the building) vs longterm (minimum, 21 days and 7 days without the onset of a case in the Lt-building facility) prophylaxis was compared using amantadine hydrochloride in the influenza seasons of 1991-1992 and 1993-1994 and rimantadine hydrochloride in the influenza season of 1994-1995. A "case" is defined as an incident of a respiratory tract illness and the isolation of an influenza virus organism. We compared the number of cases after the discontinuation of short- vs.long-term chemoprophylaxis. Prospective surveillance identified residents with new respiratory tract symptoms, and specimens for viral cultures were obtained even in the absence of temperature elevation. Results: We documented influenza A virus activity during 3 seasons (32,68, and 12 patients, respectively). During the 1991-1992, 1993-1994, and 1994-1995 influenza seasons, the patients on 11 floors were assigned to receive short-term chemoprophylaxis and those on 10 floors were assigned to long-term chemoprophylaxis. Only in 1993-1994 did chemoprophylaxis extend beyond 14 or 21 days when new cases continued beyond 14 days. Amantadine-resistant strains were circulating at that time. None of the participants in the prospective, controlled study had influenza develop after the termination of short- or long-term chemoprophylaxis. Conclusion: Antiviral chemoprophylaxis can be administered for the longer duration of 14 days or, in the absence of new culture-confirmed illness in the nursing building, for 7 days.