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Am Fam Physician. 2000;61(3):823

Amantadine and rimantadine are effective in preventing illnesses associated with type A(H3N2) and type A(H1N1) influenza viruses. A new class of antivirals, including zanamivir, which is a selective neuraminidase inhibitor, has recently been developed to decrease the duration and severity of illness related to type A and type B influenza viruses. Monto and associates evaluated the effectiveness of prophylactic zanamivir during an influenza season in two sites in the United States.

Persons between 18 and 64 years of age were eligible for the randomized, double-blinded, placebo-controlled study if they did not have any chronic medical or acute respiratory illnesses once the prophylaxis phase was begun. Prophylaxis was initiated when an increase in the incidence of influenza was documented in each location. Patients were randomized to receive 10 mg of zanamivir or placebo. Prophylaxis was given as a micronized powder through a self-activated inhalation device once daily for four weeks. Blood samples were obtained at baseline for initial evaluation, including viral serology. Patients recorded their daily temperatures and the severity of any symptoms that developed during the study period. On day 35, a second viral serology sample was obtained.

Patients were instructed to report any respiratory illness during the study period so that additional viral studies could be obtained. Infection with influenza was confirmed if influenza virus was isolated or if patients experienced a rise in titer during the prophylaxis period. Clinical influenza was defined as documented infection and two or more of the following symptoms for at least three days: cough, headache, sore throat, fever or myalgias. The primary end point was the number of cases of clinical influenza that developed in the study population.

A total of 1,107 patients was enrolled in the study, 553 in the treatment group and 554 in the placebo group. Fourteen percent of each group received an influenza vaccination before randomization. Overall, zanamivir prophylaxis was 67 percent effective in preventing clinical influenza, and 84 percent effective in preventing influenza with fever. Adverse effects were similar across groups, with approximately 75 percent of patients reporting some type of symptom.

The authors conclude that zanamivir is safe and efficacious in preventing influenza, and compliance is high, given its once-daily dosing schedule. In an accompanying editorial, Patriarca commends the study of new options for prevention of influenza but cautions against equating efficacy data with real-life effectiveness at a community level. Further studies are needed, as are improved methods of administering prophylaxis in the community.

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