Controlling Influenza: Update on Antiviral Agents
Although annual vaccination remains the most effective method for preventing seasonal influenza virus infection, antiviral medications can also help reduce the incidence of influenza and its complications. The antiviral agents available for use in the United States are oseltamivir, zanamivir, amantadine, and rimantadine. Because more than 99% of currently circulating influenza virus strains are sensitive to oseltamivir and zanamivir, these antiviral agents are recommended. Influenza A (H3N2) and 2009 H1N1 viruses are resistant to adamantanes.
The Advisory Committee on Immunization Practices has released guidelines for the use of antiviral medications to control influenza.1 Highlights of these guidelines are presented here.
INDICATIONS
Early antiviral treatment is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness or who require hospitalization. The benefits of antiviral treatment are likely to be greatest if treatment is started promptly after the onset of illness, preferably within 48 hours. However, any person with confirmed or suspected influenza who requires hospitalization should be treated, even if the patient presents more than 48 hours after the onset of illness.
Among outpatients with confirmed or suspected influenza, prompt antiviral treatment is recommended for those who are at higher risk for influenza complications on the basis of their age or underlying medical conditions. Risk factors for influenza complications are listed in Table 1. Note that although all children younger than age 5 are at elevated risk, the highest risk is for those younger than 2 years, with the highest hospitalization and death rates among infants younger than 6 months.
Treatment can be considered for outpatients with uncomplicated, suspected, or confirmed influenza who do not have known risk factors for severe or complicated illness if an antiviral can be given within 48 hours of illness onset.
DOSAGE AND DURATION OF TREATMENT
The recommended duration of antiviral treatment is 5 days. Longer treatment regimens might be necessary in severely ill hospitalized patients or persons with immunosuppression. Regimens are listed in Table 2.
CHEMOPROPHYLAXIS
Postexposure chemoprophylaxis. In randomized, controlled trials, both oseltamivir and zanamivir were effective in preventing illness among persons who received chemoprophylaxis after a household member or other close contact had laboratory-confirmed influenza.1 Because of widespread resistance among currently circulating influenza A virus strains and inherent nonsusceptibility among influenza B viruses, adamantanes have a very limited role in the prevention of influenza.
Consider antiviral chemoprophylaxis for family members or other close contacts of a person with a suspected or confirmed case who are at higher risk for influenza complications but have not been vaccinated. Unvaccinated healthcare workers who have occupational exposures and who did not use adequate personal protective equipment at the time of their exposure are also potential candidates for chemoprophylaxis.
Generally, postexposure chemoprophylaxis should be used only when antivirals can be started within 48 hours of the most recent exposure. An antiviral is typically given for a total of no more than 10 days after the most recent exposure to a close contact known to have influenza.
Preexposure chemoprophylaxis. The antiviral must be administered for the duration of time that exposure might occur. The adverse events associated with long-term use are uncertain, and prolonged use of antivirals might result in the development of resistance. Therefore, preexposure antiviral chemoprophylaxis should be reserved for persons who are at very high risk for influenza-related complications (eg, severely immunosuppressed patients) and who cannot otherwise be protected during times of high risk for exposure.
ADVERSE EFFECTS
Adverse reactions vary according to drug, dosage, and patient factors. Zanamivir can cause bronchospasm in some patients with asthma or chronic obstructive pulmonary disease. Therefore, this agent is not recommended for patients with underlying airway disease. Adverse effects reported with oseltamivir include nausea and vomiting.
1. Fiore AE, Fry A, Shay D, et al. Antiviral agents for the treatment and chemoprophylaxis of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. 2011; 60(RR01):1-24.