Influenza-related deaths can result from pneumonia as well as from exacerbations of cardiopulmonary conditions and other chronic diseases. Older adults account for ≥ 90% of deaths attributed to pneumonia and influenza. In a recent study of influenza epidemics, approximately 19,000 influenza-associated pulmonary and circulatory deaths per influenza season occurred during 1976-1990, compared with approximately 36,000 deaths per influenza season during 1990-1999. Estimated rates of influenza-associated pulmonary and circulatory deaths per 100,000 persons were 0.4-0.6 among persons aged 0-49 years, 7.5 among persons aged 50-64 years, and 98.3 among persons aged ≥ 65 years.¹ In the US, the number of influenza-associated deaths might be increasing in part because the number of older persons is increasing.^1,4 In addition, influenza seasons in which influenza A (H3N2) viruses predominate are associated with higher mortality; influenza A (H3N2) viruses predominated in 90% of influenza seasons from 1990-1999 compared with 57% of influenza seasons from 1976-1990.¹

Vaccinating persons at high risk for complications and their contacts each year before seasonal increases in influenza virus circulation is the most effective means of reducing the effect of influenza. Vaccination coverage can be increased by administering vaccine to persons during hospitalizations or routine health-care visits before the influenza season, making special visits to physicians' offices or clinics unnecessary. Vaccination of health-care workers and other persons in close contact with persons at increased risk for severe influenza illness can also reduce transmission of influenza and subsequent influenza-related complications.¹

Inactivated influenza vaccines are standardized to contain the hemagglutinins of strains (ie, typically two type A and one type B), representing the influenza viruses likely to circulate in the US in the upcoming winter. The effectiveness of influenza vaccine depends primarily on the age and immunocompetence of the vaccine recipient and the degree of similarity between the viruses in the vaccine and those in circulation. The majority of vaccinated children and young adults develop high postvaccination hemagglutination-inhibition antibody titers. These antibody titers are protective against illness caused by strains similar to those in the vaccine.¹

When the vaccine and circulating viruses are antigenically similar, influenza vaccine prevents illness in approximately 70%-90% of healthy adults aged < 65 years. Vaccination of healthy adults also has resulted in decreased work absenteeism and decreased use of health-care resources, including the use of antibiotics, when the vaccine and circulating viruses are well-matched.¹

Children aged as young as 6 months develop protective levels of antibody after influenza vaccination, although the antibody response among children at high risk for influenza-related complications might be lower than among healthy children.¹ (See PEDIATRIC USE subsection.)

Older persons aged ≥ 65 years and persons with certain chronic diseases might develop lower postvaccination antibody titers than healthy young adults and thus can remain susceptible to influenza-related upper respiratory tract infection. However, among such persons, the vaccine can be effective in preventing secondary complications and reducing the risk for influenza-related hospitalization and death among adults aged ≥ 65 years with and without high risk medical conditions (eg, heart disease and diabetes). Among elderly persons living outside of nursing homes or similar chronic-care facilities, influenza vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among elderly persons residing in nursing homes, influenza vaccine is most effective in preventing severe illness, secondary complications, and deaths. Among this population, the vaccine can be 50%- 60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death, although the effectiveness in preventing influenza illness often ranges from 30%-40%.¹

REFERENCES

1. Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2004;53: 1-40.

2. Anderson, RN, et al. National Vital Statistics Reports Vol 25 (9), 2003.

3. Thompson, WW, et al. JAMA Vol 289 (2) : 179-186, 2003.

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