INFECTION CONTROL NEWSLETTER

Broad Street Solutions
James Marx, RN, MS, CIC Editor
PO Box 16557
San Diego, CA 92176
(619)656-7887 Voice/FAX
jmarx@concentric.net 

With the prediction of influenza vaccine shortages, long term care facilities need to be ready to use the second line of defencse against an influenza outbreak, antiviral medications. The following information is taken from Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2000;49(No. RR-3).

Chemoprophylactic drugs are not a substitute for vaccination, although they are important adjuncts in the prevention and control of influenza. Both amantadine and rimantadine are indicated for the prophylaxis of influenza A infection but are not effective against influenza B. Both drugs are approximately 70%-90% effective in preventing illness from influenza A infection. When used as prophylaxis, these antiviral agents can prevent illness while permitting subclinical infection and the development of protective antibody against circulating influenza viruses. Therefore, some persons who take these drugs will develop protective immune responses to circulating influenza viruses. Amantadine and rimantadine do not interfere with the antibody response to the vaccine. Both drugs have been studied extensively in nursing home populations as a component of influenza outbreak control programs.

Zanamivir and oseltamivir have not been approved for prophylaxis, but recent community studies suggest that both drugs are similarly effective in preventing febrile, laboratory-confirmed influenza illness (efficacy: zanamivir, 84%; oseltamivir, 82%). Experience with prophylactic use of these agents in institutional settings or among patients
with chronic medical conditions is limited. Use of zanamivir has not been found to impair the immunologic response to influenza vaccine.

When determining the timing and duration for administering amantadine or rimantadine for prophylaxis, factors related to cost, compliance, and potential side ef-fects should be considered. To be maximally effective as prophylaxis, the drug must be taken each day for the duration of influenza activity in the community. However, to be most cost-effective, amantadine or rimantadine prophylaxis should be taken only during the period of peak influenza activity in a community.

Persons at High Risk Who Are Vaccinated After Influenza Activity Has Begun

Persons at high risk for complications of influenza still can be vaccinated after an outbreak of influenza has begun in a community. However, the development of antibodies in adults after vaccination can take as long as 2 weeks. When influenza vaccine is given while influenza A viruses are circulating, chemoprophylaxis with amantadine or rimantadine should be considered for persons at high risk during the time from vaccination until immunity has developed. Children who receive influenza vaccine for the first time can require as long as 6 weeks of prophylaxis (i.e., prophylaxis for 4 weeks after the first dose of vaccine and an additional 2 weeks of prophylaxis after the second dose).

Persons Who Provide Care to Those at High Risk

To reduce the spread of virus to persons at high risk during community or institutional outbreaks, chemoprophylaxis with amantadine or rimantadine during peak influenza A activity can be considered for unvaccinated persons who have frequent contact with persons at high risk. Persons with frequent contact include employees of hospitals, clinics,
and chronic-care facilities, household members, visiting nurses, and volunteer workers. If an outbreak is caused by a variant strain of influenza A that might not be controlled by the vaccine, chemoprophylaxis should be considered for all such persons, regardless of their vaccination status.

Most published reports on the use of amantadine or rimantadine to control institu-tional outbreaks of influenza A are based on studies of nursing home populations. When confirmed or suspected outbreaks of influenza A occur in institutions that house persons at high risk, chemoprophylaxis should be started as early as possible to reduce the spread of the virus. In these situations, having preapproved orders from physicians or plans to obtain orders for antiviral medications on short notice is extremely useful. When institutional outbreaks occur, chemoprophylaxis should be administered to all residents — regardless of whether they received influenza vaccine during the previous fall — and should continue for at least 2 weeks or until approximately 1 week after the end of the outbreak. The dosage for each resident should be determined individually. Chemoprophylaxis also can be offered to unvaccinated staff who provide care to persons at high risk. Prophylaxis should be considered for all employees, regardless of their
vaccination status, if the outbreak is caused by a variant strain of influenza A that is not
well matched by the vaccine.

Dosage

Dosage recommendations vary by age group and medical conditions. For persons aged 65 Years and older, the daily dose of amantadine should not exceed 100 mg for prophylaxis or treatment, because renal function declines with increasing age. For some elderly persons, the dose should be further reduced.