Pediatrics Update: "Flu" Season: Here We Go Again . . .

The "flu" season is about to begin. Are you prepared?

In this review, we provide the most recent statistics concerning the burden of this illness, summarize the newest vaccine recommendations, and discuss the potential for vaccine shortage.

Influenza is caused by orthomyxoviruses, mainly influenza A and B viruses, which invade the respiratory tract. Hemagglutinin (HA) and neuraminidase (NA) are 2 surface glycoproteins of the influenza virus that aid in viral replication. HA allows the virus to attach to the respiratory epithelial cells, and NA causes the viruses to be released and dispersed from the infected cell.¹ Point mutations that occur during viral replication lead to antigenic drift (minor antigenic variations within the same subtype) in either HA or NA.

Although antibodies to surface proteins--especially HA--develop, protection against a new variant is not guaranteed. Antigenic drift occurs continuously in both influenza A and B viruses. As a result, circulating influenza viruses are constantly changing--hence, the need to modify the vaccine developed for each influenza season. Antigenic shift (a major antigenic variation leading to the emergence of a new HA or NA) has occurred rarely only in influenza A virus. Three distinct HA subtypes (H1, H2, and H3) and 2 NA subtypes (N1 and N2) have been recognized as the cause of global epidemics.²

Less severe than the previous season, the 2004-2005 flu season in the United States spanned from October to May and peaked in mid-February.³ Influenza A (H3N2) predominated. In October 2004, it became mandatory to report deaths associated with laboratory-confirmed influenza infection in persons younger than 18 years. Between January and June 2005, 36 pediatric deaths were reported to the CDC from 16 states.³ Many of the casualties were otherwise healthy children: this emphasizes the need to prevent this illness. Nevertheless, vaccine coverage data reveal that only half of children aged 6 to 23 months, one third of high-risk children aged 2 to 17 years, and two thirds of adults 65 years or older received the influenza vaccine.⁴

In children, influenza manifests primarily with a sudden onset of systemic and respiratory symptoms. High fever, headache, malaise, myalgias, rhinitis, and cough are common findings, as are nausea, vomiting, and abdominal pain. Signs and symptoms that accompany the potential respiratory complications of influenza--such as croup, acute otitis media, bronchiolitis, secondary bacterial pneumonia, and sinusitis--may be present. Acute otitis media and lower respiratory tract disease are most common in children younger than 2 years.⁵ Nonrespiratory complications include myositis, myocarditis, encephalitis, and febrile seizures; patients may display typical features of these conditions.

Influenza can take 1 to 2 weeks or longer to resolve completely. Particularly worrisome symptoms include high fever that persists beyond the first week, respiratory distress, cyanosis, evidence of dehydration, and mental status changes.

Findings from the history and physical examination and the presence of influenza in the community provide ample support for an influenza diagnosis. However, influenza cannot be differentiated from viral syndromes caused by such pathogens as respiratory syncytial virus and parainfluenza virus based on the clinical scenario alone. A definitive diagnosis may forestall the need for additional tests in a young child and can provide the option to administer anti-flu medications while avoiding unnecessary use of antibiotics. Detection techniques for influenza A and B include viral culture, serology, rapid antigen testing, and polymerase chain reaction and immunofluorescence.⁶ In the clinic or office setting, rapid antigen tests are probably the most useful. A number of these commercial tests are available. The sensitivity and specificity of these tests vary; the site from which to obtain a specimen (eg, throat vs nasopharynx) also differs depending on the specific test, as does the type of virus detected (influenza A, influenza B, or both).

General measures. The most important treatment measures in children include maintenance of hydration and administration of analgesics and antipyretics. Salicylates are contraindicated in children with influenza because of the risk of Reye syndrome.

Antiviral agents. These drugs may be considered for children at high risk for complications--such as those with chronic pulmonary, cardiac, renal, or liver disease; immunodeficiency; hemoglobinopathy; or diabetes mellitus. Children younger than 18 years who are receiving long-term aspirin(Drug information on aspirin) therapy and all children in the first 2 years of life are considered to be at increased risk for influenza complications.⁶ Antiviral therapy may also be considered for an older and otherwise healthy child who has particularly severe symptoms.

For maximal efficacy, antiviral medications must be administered within 48 hours of symptom onset--and preferably within 30 to 36 hours of confirmation of an influenza diagnosis. Antiviral medications reduce the duration of symptoms by about 1 day.^6,7 Whether antivirals prevent influenza-related complications has not been established, however.

Table 1 provides an overview of drug prophylaxis and treatment with antiviral agents. Amantadine(Drug information on amantadine) and rimantadine are ion channel blockers; oseltamivir(Drug information on oseltamivir) and zanamivir(Drug information on zanamivir) are NA inhibitors. The ion channel blockers inhibit the transmembrane M2 protein found in influenza A virus. This protein facilitates viral replication by allowing hydrogen ions to pass into the virion.⁸ Because M2 protein is present only in influenza A virus, the ion channel blockers are not effective in the treatment of influenza B. The NA inhibitors effectively treat both influenza A and B.

Amantadine and oseltamivir are approved for use in children as young as 1 year. The inhaled agent zanamivir is approved for children 7 years and older. Rimantadine is indicated for the treatment of influenza A in patients aged 13 years and older.

The principal side effects of all the oral medications include GI disturbances, such as nausea and vomiting. The ion channel blockers may cause dizziness, jitteriness, and sleep disturbances. Zanamivir can exacerbate asthma.⁷ As with any medical intervention, the risks versus benefits of the antivirals must be evaluated for each patient.