Effect of timing of amantadine chemoprophylaxis on severity of outbreaks of influenza a in adult long-term care facilities.

BACKGROUND Long-term care facilities (LTCFs) are vulnerable to outbreaks of influenza. There are limited data on the impact of antiviral chemoprophylaxis on the duration of outbreaks of influenza. We investigated the association of timely initiation of amantadine chemoprophylaxis on the duration and severity of outbreaks of influenza A in LTCFs in New York, New York. METHODS Outbreaks of influenza A occurring from October through May each year during the period 2001-2004 in LTCFs in New York were defined as a single laboratory-confirmed case or a cluster of > or = 2 cases of influenza-like illness on a unit of an LTCF. For those facilities that provided amantadine chemoprophylaxis, we examined the association between the time to initiation of chemoprophylaxis after outbreak onset and duration of outbreak, incidence rate, and case-fatality proportion using simple t tests, multivariate analyses of covariance, and linear regression modeling. RESULTS Adjusting for influenza season year, facility bed capacity, and the proportion of residents who were vaccinated against influenza, LTCFs that initiated chemoprophylaxis 15 days after outbreak onset (25 facilities) had significantly longer duration of outbreaks (18.3 vs. 6.7 days; P < .001), higher incidence rates (10.5 cases per 100 residents vs. 6.2 cases per 100 residents; P < .023), and higher case-fatality rates (3.3 deaths per 100 residents with influenza A vs. 0.45 deaths per 100 residents with influenza A; P < .005) than did LTCFs that initiated chemoprophylaxis 5 days after outbreak onset (27 facilities). CONCLUSIONS LTCFs that initiated chemoprophylaxis >5 days after initiation of outbreaks of influenza A had significantly longer outbreaks, significantly higher incidence rates, and significantly higher case-fatality rates. These data support prompt initiation of amantadine chemoprophylaxis after identification of influenza A in LTCFs.