INFLUENZA THERAPEUTICS Amantadine (Symmetrel) and rimantadine (Flumadine) are two agents licensed in the US for persons exposed to, or infected with, influenza. Numerous studies have shown the effectiveness of both medications in reducing both the risk of unvaccinated persons becoming ill with influenza and the duration of illness in infected persons. The major limitation of these trials is that the vast majority of the subjects were healthy men in their 20's and 30's. Influenza results in 20,000-40,000 deaths per year in the US, mostly in persons over age 65 years. Though antivirals are advocated for use in high risk adults such as the elderly (1), these recommendations are based mostly on a relatively small number of observational studies in nursing homes (2).
Other important limitations of amantadine and rimantadine are that they only shorten clinical illness by several days; need to be taken within 48 hours of symptom onset; and are effective only against influenza A isolates (both influenza A and B cause clinically indistinguishable disease). Because amantadine's adverse effects (lightheadedness and falls) are particularly noxious in seniors, rimantadine is preferred in this group, despite its higher cost ($1.75/capsule vs. $0.20/capsule). Zanamivir is an recently approved neuraminidase inhibitor with activity against both type A and B influenza viruses (3). It is not well absorbed so it is given by inhalation and thus has very few adverse effects; it also reduces duration of illness by 1-3 days in infected persons and is efficacious in preventing influenza in young, healthy adults.
1. Centers for Disease Control and Prevention. Prevention and control of influenza: Recommendations of the advisory committee on immunization practices (ACIP). MMWR. 1999;48(RR-4):1-28.
2. Drinka PJ, Gravenstein S, Schilling M, Krause P, Miller BA, Shult P. Duration of antiviral prophylaxis during nursing home outbreaks of influenza A: a comparison of 2 protocols. Arch Intern Med. 1998;158(19):2155-9.
3. Monto AS, Robinson DP, Herlocher ML, Hinson JM, Jr., Elliott MJ, Crisp A. Zanamivir in the prevention of influenza among healthy adults: a randomized controlled trial. JAMA. 1999;282(1):31-5.