Although the influenza vaccine is the primary
prevention measure for influenza, antiviral drugs for influenza are
an important adjunct to influenza vaccine for the control and
prevention of influenza. Four currently licensed agents are
available in the United States: amantadine, rimantadine, zanamivir,
and oseltamivir. For detailed information on indication,
contraindication, dosing, and side effects, consult past Vidyya issues by using the Vidyya search function.
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
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
- Protection of high-risk persons vaccinated after the influenza
season has begun during the time needed to develop protective
- 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.
*The full text of the Advisory Committee on Immunization
Practices, or ACIP, Recommendations for the Prevention and Control
of Influenza, is published in the Morbidity and Mortality Weekly
Report (MMWR) April 14, 2000/Vol. 49/ No. RR-3. The MMWR
is available at the following Internet address: http://www.cdc.gov/epo/mmwr