Influenza is characterized by the sudden onset of fever, frequently with chills or rigors, headache, malaise, diffuse myalgia, and a nonproductive cough. Subsequently, the respiratory tract signs of sore throat, nasal congestion, rhinitis, and cough become more prominent. Conjunctival injection, abdominal pain, nausea, and vomiting can occur. In some children, influenza can appear as a simple upper respiratory tract infection or as a febrile illness with few respiratory tract signs. In young infants, influenza can produce a sepsis-like picture and occasionally can cause croup or pneumonia. Acute myositis characterized by calf tenderness and refusal to walk may develop after several days of influenza illness, particularly with type B infection. Reye syndrome has been associated with influenza infection, primarily with influenza B.

Etiology

Influenza viruses are orthomyxoviruses of 3 antigenic types (A, B, and C). Epidemic disease is caused by influenza virus types A and B. Influenza A viruses are subclassified by 2 surface antigens. Major changes in the predominant strain in either of these antigens, are called antigenic shifts; minor variations within the same subtypes are called antigenic drifts. Antigenic shift has occurred only with influenza A, usually at irregular intervals of 10 or more years. Antigenic drift occurs almost annually in influenza A and B viruses.

Epidemiology

Influenza is spread from person to person by inhalation of small particle aerosols, by direct contact, by large droplet infection, or by contact with articles recently contaminated by nasopharyngeal secretions. During an outbreak of influenza, the highest attack rates occur among school-age children. Secondary spread to adults and other children within the family is common. The attack rates depend in part on immunity developed by previous experience (either by natural disease or immunization) with the circulating strain or a related strain. In temperate climates, epidemics usually occur during the winter months and, within a community, peak within 2 weeks of onset and last 4 to 8 weeks or longer. Activity of 2 or 3 types or subtypes of influenza virus in a community may be associated with a prolongation of the influenza season to 3 months or more. Influenza is highly contagious, especially among institutionalized populations. Patients are most infectious during the 24 hours before the onset of symptoms and during the most symptomatic period. Viral shedding in the nasal secretions usually ceases within 7 days of the onset of illness but can be more prolonged in young children and immunodeficient patients.

The effect of influenza on immunocompetent children and children with underlying high-risk conditions is appreciable during interepidemic years and during epidemic years. Attack rates in healthy children have been estimated at 10% to 40% each year, with approximately 1% resulting in hospitalization. The risk of lower respiratory tract disease complicating influenza infection in children, primarily pneumonia, croup, wheezing, and bronchiolitis, has ranged from 0.2% to 25%. A wide spectrum of complications, such as Reye syndrome, myositis, and central nervous system (CNS) manifestations, can occur. The risk of Reye syndrome, which occurs primarily in school-age children, has decreased during recent years. Respiratory tract viruses other than influenza (eg, respiratory syncytial virus and parainfluenza viruses) produce life-threatening illness more commonly in young children than in adults, and, as a result, morbidity and mortality rates in children for influenza and the effect of control measures are more difficult to determine.

Excess rates of hospitalization have been documented for children with influenza, including neonates and children up to 5 years of age and children who have hemoglobinopathies, bronchopulmonary dysplasia, asthma, cystic fibrosis, malignant neoplasms, diabetes mellitus, or chronic renal disease. Pulmonary complications, such as bronchitis and pneumonia, seem to be more common among these children. Influenza in neonates has been associated with considerable morbidity, including a sepsis-like syndrome, apnea, and lower respiratory tract disease. Studies have shown that children younger than 5 years of age have the second highest rate of hospitalization with influenza, only exceeded by persons older than 65 years of age.

Influenza Pandemics
When the circulating strain of virus is substantially different from that occurring in other years (ie, antigenic shift), more severe epidemics or pandemics can occur with excess morbidity and mortality. During the 20th century, there were 3 influenza pandemics, including the one in 1918 that killed more than 20 million people worldwide, many of whom were young adults. The mortality rates with the more recent pandemics of 1957 and 1968 were reduced, in part, through the use of antimicrobial therapy for secondary bacterial infections and more aggressive supportive care. Experience indicates that these pandemics occur at irregular intervals and have the potential to be true public health emergencies. Dealing with the next influenza pandemic will require extensive use of vaccine and antiviral and antibacterial agents and development of triage policies for hospital and intensive care utilization. An interagency group, which includes members of the American Academy of Pediatrics, is preparing a plan for the public health response to the next pandemic.

The incubation period usually is 1 to 3 days.

Control of fever with acetaminophen or other appropriate antipyretics may be important in young children because the fever and other symptoms of influenza could exacerbate underlying chronic conditions. Children and teenagers with influenza should not receive salicylates because of the resulting increased risk of developing Reye syndrome.

Control Measures

Influenza Vaccine
The inactivated influenza vaccines produced in embryonated eggs are immunogenic and associated with minimal side effects. These multivalent vaccines contain 3 virus strains (usually 2 type A and 1 type B) with composition changed periodically in anticipation of the prevalent influenza strains expected to circulate in the United States in the upcoming winter. Vaccines include inactivated whole-virus vaccine prepared from the intact purified virus particles, the subvirion vaccine prepared by the additional step of disrupting the lipid-containing membrane of the virus, and purified surface-antigen vaccine.

Immunogenicity in Children
Children younger than 9 years of age who have little experience with influenza require 2 doses of vaccine administered 1 month apart to produce a satisfactory antibody response. Children previously primed with a related strain of influenza by infection or immunization exhibit a brisk antibody response to 1 dose of the vaccine.

Vaccine Efficacy
The effect of influenza immunization on acute respiratory tract illness is less likely to be evident in pediatric than adult populations because of the frequency of upper respiratory tract infections and influenza-like illness caused by other viral agents in young children. Protection in healthy subjects usually is 70% to 80%, with a range of 50% to 95% varying with the closeness of vaccine strain match to the wild strain. The duration of protection is brief, usually presumed to be less than 1 year. Efficacy has not been evaluated in infants immunized during the first 6 months of life.

Recommendations for Influenza Immunization
Annual influenza vaccine can be given to infants older than 6 months of age, children, adolescents, and adults to reduce the impact of influenza. Priority should be given to targeted high-risk groups.

Targeted High-Risk Children and Adolescents
Yearly influenza immunization, administered during the autumn, is recommended for children 6 months of age and older syndrome following influenza

Influenza vaccine may be administered to any healthy child or adolescent who wishes to reduce the chance of becoming infected with influenza. The morbidity from influenza among healthy children can be appreciable. Influenza vaccine does not adversely affect the safety of breastfeeding for mothers or infants; therefore, breastfeeding is not a contraindication for immunization.

Vaccine Administration
Influenza vaccine should be administered during the autumn of each year before the start of the influenza season at the time specified in the yearly recommendations of the CDC Advisory Committee on Immunization Practices.

Annual immunization is recommended because of declining immunity during the year after immunization and because in most years, at least one of the antigens is changed to increase the antigenic similarity between the vaccine and circulating strains.

Influenza vaccine may be administered simultaneously (but at a separate site and with a different syringe) with other routine immunizations in children.

Reactions, Adverse Effects, and Contraindications
Inactivated influenza vaccine contains only noninfectious viruses and cannot cause influenza. Fever occurs primarily 6 to 24 hours after immunization in children younger than 24 months of age. Local reactions are infrequent in children younger than 13 years of age. In children 13 years of age or older, local reactions occur in approximately 10% after immunization.

Immunization of children who have asthma or cystic fibrosis with the currently available influenza vaccines is not associated with a detectable increase in adverse reactions or exacerbations.

Children demonstrating severe anaphylactic reaction to chickens or egg protein can experience, on rare occasions, a similar type of reaction to killed influenza vaccines. Although influenza vaccine has been administered safely to such children after skin testing and even desensitization, these children generally should not receive influenza vaccine because of their risk of reactions, the likely need for yearly immunization, and the availability of chemoprophylaxis against influenza infection.

Related Links:
Flu Vaccine

CDC Influenza Website