Introduction
Epidemics of influenza typically occur during the winter
months and have been responsible for an average of approximately
36,000 deaths/year in the United States during 1990–1999.
Influenza viruses also can cause pandemics, during which
rates of illness and death from influenza-related complications
can increase dramatically worldwide. 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.
Influenza vaccination is the primary method for preventing
influenza and its severe complications. In this report from
the Advisory Committee on Immunization Practices (ACIP),
the primary target groups recommended for annual vaccination
are 1) groups that are at increased risk for influenza-related
complications (e.g., persons aged > 65 years and
persons of any age with certain chronic medical conditions);
2) the group 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 (e.g., 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. Although influenza
vaccination levels increased substantially during the 1990s,
further improvements in vaccine coverage levels are needed,
chiefly among persons aged <65 years who are at increased
risk for influenza-related complications among all racial
and ethnic groups and among blacks and Hispanics aged > 65
years. ACIP recommends using strategies to improve vaccination
levels, including using reminder/recall systems and standing
orders programs. Although influenza vaccination remains the
cornerstone for the control and treatment of influenza, information
is also presented regarding antiviral medications, because
these agents are an adjunct to vaccine.
Biology of Influenza
Influenza A and B are the two types of influenza viruses
that cause epidemic human disease. Influenza A viruses are
further categorized into subtypes on the basis of two surface
antigens: hemagglutinin (H) and neuraminidase (N). Influenza
B viruses are not categorized into subtypes. 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. Both influenza A and B viruses
are further separated into groups on the basis of antigenic
characteristics. New influenza virus variants result from
frequent antigenic change (i.e., antigenic drift) resulting
from point mutations that occur during viral replication.
Influenza B viruses undergo antigenic drift less rapidly
than influenza A viruses.
A person's immunity to the surface antigens, including hemagglutinin,
reduces the likelihood of infection and severity of disease
if infection occurs. Antibody against one influenza virus
type or subtype confers limited or no protection against
another. Furthermore, antibody to one antigenic variant of
influenza virus might not protect against a new antigenic
variant of the same type or subtype. Frequent development
of antigenic variants through antigenic drift is the virologic
basis for seasonal epidemics and the reason for the usual
incorporation of > 1 new strains in each year's
influenza vaccine.
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