Amantadine and rimantadine are indicated for the prophylaxis and treatment of influenza A infection. Zanamivir and oseltamivir are neuraminidase inhibitors with activity against both influenza A and B viruses. Both zanamivir and oseltamivir were approved in 1999 for treatment of uncomplicated influenza, but neither is approved for prophylaxis.

When administered prophylactically to healthy adults or children, both amantadine and rimantadine are approximately 70%-90% effective in preventing illness from influenza A infection. Chemoprophylaxis is not a substitute for vaccination, but may be considered for the following situations.

• Protection of high-risk persons vaccinated after the influenza season has begun during the time needed to develop protective antibody levels
• Protection of persons who cannot receive influenza vaccine
• Use by unvaccinated close contacts of high-risk persons during the peak of the influenza season in order to reduce the chances of spreading influenza to high-risk persons
• Protection of immunocompromised persons who may not develop protective levels of antibody following vaccination
• Outbreak control in an institution housing or caring for high-risk people

When amantadine or rimantadine is administered as prophylaxis, factors related to cost, compliance, and potential side effects should be considered when determining the period of prophylaxis.

When administered as treatment within 2 days of illness onset in healthy adults, amantadine and rimantadine can reduce the severity and duration of signs and symptoms of influenza A illness, and zanamivir and oseltamivir can reduce the duration of uncomplicated influenza A and B illness by approximately one day. To reduce the emergence of antiviral drug-resistant viruses, amantadine or rimantadine treatment should be discontinued as soon as clinically warranted, generally 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.

Both amantadine and rimantadine can cause central nervous system (CNS) and gastrointestinal side effects. However, the incidence of CNS side effects (e.g., nervousness, anxiety, difficulty concentrating, and lightheadedness) is higher among persons taking amantadine than among those taking rimantadine. Dosages of both drugs should be lower for older persons and persons with reduced kidney function. Consult the package insert for more information on dosing and side effects.

Zanamivir is given as an inhaled powder. Among persons with chronic lung disease, such as asthma or chronic obstructive pulmonary disease, inhalation of zanamivir may result in difficulty breathing. If, after considering the potential risks and benefits, this drug is prescribed for someone with chronic respiratory disease, the drug should be used with caution. Precautions should include proper monitoring and availability of supportive care, including availability of short-acting brochodilators.

The primary side effects associated with oseltamivir use are nausea and vomiting. These side effects may be diminished if the drug is taken with food.