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.
A person's immunity to the surface antigens, especially hemagglutinin, reduces
the likelihood of infection and severity of disease if infection occurs. Antibody
against one influenza virus type or subtype confers little or no protection
against another virus. 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 virological basis for seasonal epidemics and the reason for the usual
incorporation of one or more new strains in each year's influenza vaccine.1
Formal subclassification utilizing neuraminidase antigens has not been done for influenza B viruses.
The incubation period for influenza is 1- 4 days with an average of 2 days.
Adults typically are infectious from the day before symptoms begin through approximately
5 days after illness onset. Children can be infectious for ≥ 10 days, and
young children can shed virus for ≤ 6 days before their illness onset.
Severely immunocompromised persons can shed virus for weeks or months.
Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (eg, fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis). Among children, otitis media, nausea and vomiting are also commonly reported with influenza illness.
Influenza illness typically resolves after a limited number of days for the
majority of persons, although cough and malaise can persist for > 2 weeks.
Among certain persons, influenza can exacerbate underlying medical conditions
(eg, pulmonary or cardiac disease), lead to secondary bacterial pneumonia or
primary influenza viral pneumonia, or occur as part of a coinfection with other
viral or bacterial pathogens. Young children with influenza infection can have
initial symptoms mimicking bacterial sepsis with high fevers and ≤ 20% of
children hospitalized with influenza can have febrile seizures. Influenza infection
has also been associated with encephalopathy, transverse myelitis, Reye syndrome,
myositis, myocarditis, and pericarditis.1
The risks for complications, hospitalizations, and deaths from influenza are
higher among persons aged ≥ 65 years, young children, and persons of any age
with certain underlying health conditions than among healthy older children
and younger adults.1 Among children aged 0 - 4 years, hospitalization
rates have ranged from approximately 500/100,000 children for those with high-risk
medical conditions to 100/100,000 children for those without high-risk medical
conditions, and are comparable to rates reported among persons aged ≥ 65 years.1
In addition, influenza is a leading cause of death in young children and, along
with pneumonia, is the sixth leading cause of death in those 1-4 years of age.2
During influenza epidemics from 1969-1970 through 1994-1995, the estimated
overall number, for all ages, of influenza-associated hospitalizations in the
US has ranged from approximately 16,000 to 220,000/epidemic. An average of approximately
114,000 influenza-related excess hospitalizations occurred per year, with 57%
of all hospitalizations occurring among persons aged < 65 years. Since the
1968 influenza A (H3N2) virus pandemic, the greatest numbers of influenza-associated
hospitalizations have occurred during epidemics caused by type A (H3N2) viruses,
with an estimated average of 142,000 influenza-associated hospitalizations per
year.1
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.
Last updated on RxList: 10/8/2007