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Medical Letter |
Interim Vaccine Recommendations
Currently available inactivated influenza vaccine should be reserved for adults > 65 years old, children 6–23 months of age, residents of nursing homes and long-term care facilities, persons 2–64 years old with chronic medical conditions (such as cardiovascular or respiratory disease, diabetes or immunosuppression), children 6 months to 18 years old on chronic aspirin therapy, and women who will be pregnant during the influenza season. In addition, health care workers involved in direct patient care and caretakers of children < 6 months of age should be immunized; if they are 5–49 years old and not pregnant, these individuals (except for health care workers who care for severely immunocompromised patients) could be immunized with the intranasal live-attenuated vaccine (FluMist).
Indications for Prophylaxis
Prophylaxis with an antiviral drug may be useful for the unvaccinated, or if an influenza outbreak occurs before or less than 2 weeks after vaccination with inactivated vaccine, or if circulating strains prove to be different from vaccine strains. Antiviral drugs could interfere with the efficacy of the live-virus FluMist vaccine; they should not be started for at least 2 weeks after and should be stopped at least 48 hours before FluMist administration.
Amantadine and Rimantadine
Started before exposure and continued throughout the period of exposure (up to 6–8 weeks), oral amantadine (Symmetrel, and others) or rimantadine (Flumadine, and others) can prevent illness due to influenza A in 70–90% of patients. Treatment begun within 48 hours after the onset of illness decreases the duration of fever and symptoms. Both drugs can be used for prophylaxis and treatment of patients at least 12 months old; neither is active against influenza B.
Insomnia, lightheadedness, nervousness, difficulty concentrating, delirium, hallucinations and seizures can occur in patients taking amantadine at recommended doses. These symptoms are more common in the elderly and with concurrent use of anticholinergics or older antihistamines such as diphenhydramine (Benadryl, and others) that have anticholinergic effects. Rimantadine also has CNS adverse effects, but they occur less frequently than with amantadine.
Resistance to amantadine and rimantadine can occur rapidly, and resistant virus can be transmitted to the close contacts of patients treated with these drugs.3
Neuraminidase Inhibitors
Oseltamivir (Tamiflu) and zanamivir (Relenza) are about 70–90% effective for pre- and post-exposure prophylaxis in households with either influenza A or B.4,5 Started within 36–48 hours after the onset of illness, they can decrease the severity and duration of symptoms.6 Oseltamivir treatment of proven influenza illness has been shown to lower the incidence of influenza-related lower respiratory tract complications and hospitalization.7
Adverse effects of oseltamivir include nausea, vomiting and headache. Zanamivir, which is administered as an inhaled powder and is FDA-approved only for treatment, can cause cough, nasal and throat discomfort, bronchospasm and decreased lung function; it should generally not be used in patients with asthma or chronic obstructive pulmonary disease.8
Resistance to either drug can occur, but appears to be uncommon in immunocompetent patients being treated for acute influenza.9 Influenza A viral strains resistant to amantadine and rimantadine generally remain susceptible to neuraminidase inhibitors.
Avian Flu
No commercial influenza vaccine protects against the pathogenic strains of avian influenza (H5N1, H7N2, H9N2, H7N3, H7N7) that have caused disease in humans in recent years.10,11 Most cases have occurred in patients who had close contact with infected poultry, and the H5N1 outbreak has been confined to Southeast Asia. The CDC recommends that travelers to countries in Asia with documented outbreaks avoid live poultry markets, farms, and contact with surfaces that appear to be contaminated with poultry feces, and eat poultry products only if well cooked. Travelers should wash their hands frequently with soap and water or use an alcohol-based hand rub.12 Neuraminidase inhibitors have been effective against some avian strains of influenza in animal studies and are options for prophylaxis and treatment of suspected H5N1 disease.13,14 Amantadine and rimantadine have not been effective against recent H5N1 strains in humans, but may be effective against other avian species.
Conclusion
Vaccination is the best way to prevent influenza, but this year it will not be available to everyone. Antiviral drugs can also prevent influenza when taken during outbreaks in institutions or after direct exposure. The same drugs can be effective for treatment of acute influenza. Oseltamivir (Tamiflu) offers the best combination of effectiveness and safety. Generic amantadine is the least expensive, but is only active against influenza A and has troublesome CNS effects, especially in the elderly. Rimantadine has less CNS toxicity.
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Footnotes
The following articles from The Medical Letter have been selected on the basis of their importance to obstetrician–gynecologists. For subscription information, go to http://medicalletter.org/tml/obgyn
doi:10.1097/01.AOG.0000153711.21103.16
References
1. Interim Influenza Vaccination Recommendations, 2004–05 Influenza Season. Available at www.cdc.gov/flu.
2. Influenza Vaccine 2004–2005. Med Lett Drugs Ther 2004;46:83.[Medline]
3. Shiraishi K, et al. High frequency of resistant viruses harboring different mutations in amantadine-treated children with influenza. J Infect Dis 2003;1:57–61.
4. Hayden FG, et al. Management of influenza in households: a prospective, randomized comparison of oseltamivir treatment with or without postexposure prophylaxis. J Infect Dis 2004;189:440.[Medline]
5. Oxford J, et al. A new millennium conundrum: how to use a powerful class of influenza anti-neuraminidase drugs (NAIs) in the community. J Antimicrob Chemother 2004;53:133.
6. Cooper NJ, et al. Effectiveness of neuraminidase inhibitors in treatment and prevention of influenza A and B: systematic review and meta-analyses of randomised controlled trials. BMJ 2003;7:1235.
7. Kaiser L, et al. Impact of oseltamivir treatment on influenzarelated lower respiratory tract complications and hospitalizations. Arch Intern Med 2003;163:1667.
8. JC Williamson and PS Pegram. Neuraminidase inhibitors in patients with underlying airways disease. Am J Respir Med 2002;1:85.[Medline]
9. A Moscona. Oseltamivir-resistant influenza? Lancet 2004;364:733.[Medline]
10. Koopmans M, et al. Transmission of H7N7 avian influenza A virus to human beings during a large outbreak in commercial poultry farms in the Netherlands. Lancet 2004;363:587.[Medline]
11. Tran TH, et al. Avian influenza A (H5N1) in 10 patients in Vietnam. N Engl J Med 2004;350:1179.
12. Advice for Travelers. Treatment Guidelines from The Medical Letter 2004;2:33.[Medline]
13. Leneva IA, et al. Efficacy of zanamivir against avian influenza A viruses that possess genes encoding H5N1 internal proteins and are pathogenic in mammals. Antimicrob Agents Chemother 2001;45:1216.
14. Leneva IA, et al. The neuraminidase inhibitor GS4104 (oseltamivir phosphate) is efficacious against A/Hong Kong/156/97 (H5N1) and A/Hong Kong/1074/99 (H9N2) influenza viruses. Antiviral Res 2000;48:101.[Medline]
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