Antiviral treatment and chemoprophylaxis Although inﬂuenza vaccination is the cornerstone for the control and treatment of inﬂuenza, antiviral medications can be used as an adjunct to vaccination. Four antiviral drugs are available for the prevention and treatment of inﬂuenza infec- tions (Table 3). Oseltamivir and zanamivir are neuraminidase inhibitors, acting against both inﬂuenza A and B infections. Rimantadine and amantadine block the M2 channel of inﬂu- enza A virus. Dosage recommendations vary by age group and medical conditions (Table 3). When administered within two days of illness onset to healthy adults, amantadine and rimantadine can reduce the duration of uncomplicated inﬂuenza A illness, and zanamivir and oseltamivir can reduce the duration of uncomplicated inﬂuenza A and B illness by approximately 1 day, compared with placebo (Harper et al 2005). Antiviral therapies could possibly reduce inﬂuenza duration and complications in patients with COPD and other medical conditions, although information concerning the effectiveness of amantadine, rimantadine, zanamivir, and oseltamivir in patients with chronic medical conditions are limited (Hayden et al 1997; Cooper et al 2003; Jefferson et al 2000). Resistance against rimantadine and amantadine can ap- pear rapidly in approximately one third of patients receiving these therapies (Hayden et al 1992; Saito et al 2002). During the course of amantadine or rimantadine therapy, resistant inﬂuenza strains can replace susceptible strains within 2–3 days of starting therapy. Resistant viruses have been isolated from persons who live at home or in an institution where other residents are taking or have recently taken amantadine or rimantadine as therapy. Amantadine- and rimantadine- resistant viruses are not more virulent or transmissible than Preventing inﬂuenza in COPD susceptible viruses (Hayden et al 1992). The screening of epidemic strains of inﬂuenza A has rarely detected amanta- dine- and rimantadine-resistant viruses (Ziegler et al 1999). Development of viral resistance to zanamivir and oseltamivir during treatment has been identiﬁed but does not appear to be frequent (Jackson et al 2000). To reduce the emergence of antiviral drug-resistant viruses, amantadine or rimantadine therapy for persons with inﬂuenza A illness should be discon- tinued as soon as clinically warranted, typically after 3–5 days of treatment or within 24–48 hours after the disappearance of signs and symptoms. The recommended duration of treatment with either zanamivir or oseltamivir is 5 days. Chemoprophylactic drugs can be used as adjuncts to vac- cination in preventing and controlling inﬂuenza. Both aman- tadine and rimantadine are indicated for chemoprophylaxis of inﬂuenza A infection, but not inﬂuenza B (Table 3). Both drugs are approximately 60%–90% effective in preventing illness from inﬂuenza A infection (Wintermeyer and Nahata 1995; Demicheli et al 2000). Amantadine and rimantadine have been studied extensively among nursing home popula- tions as a component of inﬂuenza outbreak-control programs, which can limit the spread of inﬂuenza within chronic-care institutions (Guay 1994; Wintermeyer and Nahata 1995). Antiviral therapy can be used to prevent inﬂuenza in patients with COPD and other chronic conditions in the fol- lowing situations: 1) When they have not been vaccinated. 2) When inﬂuenza vaccine has a lower efﬁcacy and effec- tiveness due to viral antigenic variation. 3) When inﬂuenza vaccine is contraindicated. 4) When they are known to have experienced Guillain-Barré syndrome within 6 weeks after a previous inﬂuenza vac- cination. Table 3 Comparison of antiviral drugs for the treatment and prophylaxis of inﬂuenza Dosage in individuals Effective aged >12 years against Drug inﬂuenza Amantadine A Side effects % 0–70 Indication Treatment 100 mg bid* Prophylaxis 100 mg bid* Rimantadine Treatment 100 mg bid* A Prophylaxis 100 mg bid* Oseltamivir Treatment 75 mg bid A and B Prophylaxis 75 mg/day Zanamivir Treatment 10 mg bid A and B Prophylaxis not approved Abbreviations: bid, twice per day; COPD, chronic obstructive pulmonary disease. Notes: *100 mg/day in the elderly. 0–2 5–10 <1 Precautions in patients with: COPD No Renal problems Yes No No Yes Yes Yes No Hepatic problems Yes Yes No No International Journal of COPD 2007:2(1) 49