Influenza is a significant cause of death and, along with pneumonia, is the seventh leading cause of death across generations.² This understates the actual impact of influenza as the complications associated with influenza infection are also categorized as heart disease, chronic lower respiratory disease, or diabetes.³ As a result, influenza each year conservatively contributes to over 36,000 deaths,¹ many of which could be prevented through vaccination. Influenza viruses also can cause pandemics during which rates of illness and death from influenza-related complications can increase dramatically. Influenza viruses cause disease among all age groups. Rates of infection are highest among children, but rates of serious illness and death are highest among persons aged ≥ 65 years and persons of any age who have medical conditions that place them at increased risk for complications from influenza.¹

Influenza vaccination is the primary method for preventing influenza and its severe complications. The primary target groups recommended for annual vaccination are 1) groups that are at increased risk for influenza-related complications (eg, persons aged ≥ 65 years, children aged 6 - 23 months, pregnant women, and persons of any age with certain chronic medical conditions); 2) persons aged 50 - 64 years because this group has an elevated prevalence of certain chronic medical conditions; and 3) persons who live with or care for persons at high risk (eg, health-care workers and household contacts who have frequent contact with persons at high risk and who can transmit influenza to persons at high risk). Vaccination is associated with reductions in influenza-related respiratory illness and physician visits among all age groups, hospitalization and death among persons at high risk, otitis media among children, and work absenteeism among adults.¹

Among persons aged ≥ 65 years, influenza vaccination levels increased from 33% in 1989 to 66% in 1999, surpassing the Healthy People 2000 goal of 60%. Although estimated influenza vaccination coverage for the 1999 - 2000 season reached the highest levels recorded among older black, Hispanic, and white populations, vaccination levels among blacks and Hispanics continue to lag behind those among whites.¹

Increasing vaccination coverage among persons who have high-risk conditions and are aged < 65 years, including children at high risk, is the highest priority for expanding influenza vaccine use.¹

Vaccination of health-care workers has been associated with reduced work absenteeism and fewer deaths among nursing home patients. Efforts should be made to educate health-care workers regarding the benefits of vaccination and the potential health consequences of influenza illness for themselves and their patients.¹

Influenza A and B are the two types of influenza viruses that cause epidemic human disease.¹ Influenza A viruses are further categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N). Influenza B viruses are not categorized into subtypes. Both influenza A and B viruses are further separated into groups based on antigenic characteristics. New influenza virus variants result from frequent antigenic change (ie, antigenic drift), resulting from point mutations that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have been in global circulation. In 2001, influenza A (H1N2) viruses that probably emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses began circulating widely.

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