Are You Ready for This Year's Influenza Season?

According to the CDC, last year's influenza season in the United States was mild to moderate.¹Influenza activity increased in mid January and peaked during mid to late February. The percentage of deaths associated with pneumonia and influenza exceeded the epidemic threshold for 5 consecutive weeks. Influenza A (H3N2) viruses predominated, although toward the end of the season, influenza B viruses were identified more often than influenza A viruses.

This year's influenza season is approaching fast. The report from the CDC's Advisory Committee on Immunization Practices (ACIP) contains several notable changes with respect to its recommendations for the prevention and treatment of influenza.² These changes can be summarized as follows:

The optimal time to receive the influenza vaccine is during October and November. However, because of the vaccine distribution delays that occurred during the past 2 years, the ACIP recommends that vaccination efforts in October focus on persons at greatest risk for influenza-related complications and on health care workers. Vaccination of the other groups should begin in November.

Vaccination efforts for all groups should continue into December and later, for as long as the vaccine is available.

Because young children are at increased risk for influenza-related hospitalization, the vaccination of healthy children aged 6 to 23 months is encouraged. However, a full recommendation for vaccination of this group has not yet been made.

The 2002-2003 trivalent influenza vaccine contains A/Moscow/10/99-like (H3N2), A/New Caledonia/20/ 99-like (H1N1), and B/Hong Kong/ 330/2001-like strains.

A limited amount of influenza vaccine with reduced thimerosal content will be available for the 2002-2003 influenza season.

Highlights of the ACIP's latest recommendations follow.

CANDIDATES FOR THE VACCINE

Influenza causes approximately 20,000 deaths per year in the United States.^3,4 The hardest hit are the elderly and patients who have underlying medical conditions. Although vaccination rates have improved in persons aged 65 years and older, the influenza vaccine continues to be underused in certain groups. For example, in 1997, the vaccine was given to fewer than 30% of high-risk persons younger than 65 years⁵ and to only about 34% of health care workers.⁶

The influenza vaccine is the primary method of preventing influenza and its serious complications. Influenza vaccination is recommended for persons who are at increased risk for complications from influenza or for those in whom the prevalence of chronic medical conditions that place them at risk for such complications is higher (Table 1).² The risks of influenza-related complications, hospitalizations, and deaths are increased in persons aged 65 years and older, very young children, and persons with underlying medical conditions.

The vaccine also is recommended for persons aged 50 to 64 years, because 24% to 32% of this age group have 1 or more chronic medical conditions that place them at high risk for influenza-related hospitalization and mortality. Influenza immunization is also recommended for persons who can transmit infection to high-risk persons and for women who will be in the second or third trimester of pregnancy during the influenza season.

For children who are aged 6 to 23 months, vaccination is encouraged when feasible, because these children are at increased risk for influenza-related hospitalizations.² However, a full recommendation has not yet been made, pending review by the ACIP, the American Academy of Pediatrics, and the American Academy of Family Physicians.

WHEN TO GIVE THE VACCINE

The best time to administer the influenza vaccine is typically during October and November. However, because of vaccine distribution delays in the last 2 influenza seasons, the ACIP recommends that vaccination efforts in October (and earlier) be focused on high-risk persons and on health care workers.² Vaccination of children younger than 9 years who are receiving the vaccine for the first time should begin in October also, since they need a booster dose 1 month after the initial dose. Vaccination of other groups (such as household members of persons at high risk) should begin in November.

To minimize the chance that large, organized vaccination campaigns will have to be canceled because the vaccine is not available, persons planning such campaigns may consider scheduling these events after mid October. Administering the vaccine before October should generally be avoided in facilities such as nursing homes, because antibody levels can begin to decline within a limited time after vaccination.⁷

THIS YEAR'S VACCINE

The virus strains that are contained in the 2002-2003 trivalent influenza vaccine are A/Moscow/10/99-like (H3N2), A/New Caledonia/20/ 99-like (H1N1), and B/Hong Kong/ 330/2001-like strains. The specific dosage recommendations are shown in Table 2.

Electron micrographs revealing the influenza virus are shown in the Figure.

ANTIVIRAL AGENTS

Although antiviral agents should not be used as a substitute for influenza vaccination, these drugs can be a useful adjunct. Four agents are currently licensed and available in the United States for the treatment of influenza: amantadine, rimantadine, zanamivir, and oseltamivir (Table 3).

Amantadine and rimantadine are active against influenza A viruses but not against influenza B viruses. Amantadine has been approved for prophylaxis and treatment in adults and children. Rimantadine has been approved for prophylaxis and treatment in adults and for prophylaxis in children; some infectious disease experts consider rimantadine to be appropriate for treatment of influenza in children.⁸

Zanamivir and oseltamivir are neuraminidase inhibitors and are active against both influenza A and B viruses. They have been approved for the treatment of uncomplicated influenza. Zanamivir has been approved for treatment of persons aged 7 years and older, and oseltamivir has been approved for treatment of persons 1 year and older. Oseltamivir has also been approved for prophylaxis in persons 13 years and older.

Amantadine, rimantadine, and oseltamivir are given orally. Amantadine and rimantadine are available in tablet or syrup form, and oseltamivir is available as a capsule. Zanamivir is available as a dry powder that is self-administered via oral inhalation.

Treatment: When given within 2 days of the onset of illness to otherwise healthy adults, amantadine and rimantadine can shorten the duration of uncomplicated illness caused by influenza A. Zanamivir and oseltamivir can shorten the duration of uncomplicated illness caused by influenza A or B virus by about 1 day.^9,10

To reduce the emergence of drug-resistant viruses, amantadine and rimantadine therapy should be discontinued as soon as clinically warranted-usually after 3 to 5 days of treatment or within 24 to 48 hours after the disappearance of signs and symptoms. The currently recommended duration of therapy with zanamivir or oseltamivir is 5 days.

Prophylaxis: Amantadine and rimantadine are about 70% to 90% effective in preventing illness from influenza A virus infection.^9,10 To be maximally effective as prophylaxis, amantadine or rimantadine must be taken each day for the duration of influenza activity in the community; however, there is evidence that to be most cost-effective, these agents should be taken only during the period of peak influenza activity in a community.¹¹ n

References:

REFERENCES:

1. Centers For Disease Control and Prevention. Update: influenza activity-United States and worldwide, 2001-02 season, and composition of the 2002-03 influenza vaccine. MMWR. 2002;51:503-506.

2. Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. 2002;51(RR-3):1-31.

3. Simonsen L, Schonberger LB, Stroup DF, et al. The impact of influenza on mortality in the USA. In:

Brown LE, Hampson AW, Webster RG, eds. Options

for the Control of Influenza III: Proceedings of the 3rd International Conference on Options for the Control of Influenza, Cairns, Australia, 4-9 May 1996. Amsterdam: Elsevier Science; 1996:26-33.

4. Lui KJ, Kendal AP. Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. Am J Public Health. 1987;77:712-716.

5. Kramarz P, DeStefano F, Garguillo P, Chen R, for the Vaccine Safety Datalink Team. Determinants of influenza vaccination in children with asthma in four health maintenance organizations (abstract). 1998 Epidemic Intelligence Service Conference. Atlanta: Centers for Disease Control and Prevention; 1998.

6. Walker FJ, Singleton JA, Lu PJ, Strikas RA. Influenza vaccination of health care workers in the United States, 1989-1997. Infect Control Hosp Epidemiol. 2000;21:113.

7. McElhaney JE, Gravenstein S, Upshaw CM, et al. Immune response to influenza vaccination in institutionalized elderly: effect of different T-cell subsets. Vaccine. 1998;16:403-409.

8. American Academy of Pediatrics. Influenza. In: Pickering LK, ed. 2002 Redbook: Report of the Committee on Infectious Diseases. 25th ed. Grove Village, Ill: American Academy of Pediatrics; 2000:351-359.

9. Demicheli V, Jefferson T, Rivetti D, Deeks J. Prevention and early treatment of influenza in healthy adults. Vaccine. 2000;18:957-1030.

10. Tominack RL, Hayden FG. Rimantadine hydrochloride and amantadine hydrochloride use in influenza A virus infections. Infect Dis Clin North Am. 1987;1:459-478.

11. Patriarca PA, Arden NH, Koplan JP, Goodman RA. Prevention and control of type A influenza infections in nursing homes: benefits and costs of four approaches using vaccination and amantadine. Ann Intern Med. 1987;107:732-740.