Annually, during the winter months emergency departments (EDs) experience higher patient volumes attributed to viral illnesses. In temperate climates across the northern hemisphere, this is the season associated with influenza infection, with peak activity between December and February. An average of 10% to 40% of healthy children are infected with influenza every season.^1,2 This rate varies slightly based on strain prevalence and vaccine efficacy. About 1 in 1000 children infected with influenza will require hospitalization, with those less than 6 months old requiring admission at a greater rate.^2–4 Death rates vary annually based on the severity of the strain, and are highest among adults greater than 65. On average, 100 children die in the United States from influenza infection every year, with 0.2 deaths for every 100,000 children. In comparison, there are typically 5000 flu-related fatalities each year in adults, with 1.5 deaths per 100,000 people of age between 19 and 65 years, and even higher rates among those above 65 years of age.^5,6

Adults and adolescents will present with “classic” flu symptoms, including sudden onset of fever, malaise, headache, pharyngitis, and myalgias. Children may present with a wider array of symptoms, especially abdominal complaints such as nausea, vomiting, diarrhea, and abdominal pain, while infants can present with viral sepsis, croup, or bronchiolitis. In all pediatric populations, fever and cough are the most common symptoms associated with influenza infection (see Table 64-1).^7–12 Once exposed to the virus, replication occurs in the respiratory epithelium. It is then spread via respiratory droplets, either by direct transmission to a person or from a contaminated surface. After exposure, it can take anywhere from 1 to 4 days (on average, 2) for symptoms to develop. The general population is contagious 1 day prior and up to 1 week after symptom onset. However, children and immunocompromised hosts can be contagious for longer periods, with viral shedding lasting up to 14 days.

Acute otitis media (AOM) is the most common complication, affecting 20% of children with influenza. With influenza vaccination, rates of AOM can be reduced by 30%, thereby reducing antibiotic use.^13–15 Bacterial pneumonia is a less common but important complication of influenza, affecting 2% to 4% of children infected with influenza. Children with pneumonia will typically present with worsening or late rise of fever, tachypnea, and oxygen requirement. Less common complications, but notable secondary to their potential for associated morbidity, include myocarditis, myositis, and encephalitis, with myocarditis occurring more frequently attributed to Influenza B in recent years.^16–20 Timely vaccination can reduce the risk of mortality in high-risk patients by 50%, and by 65% in healthy children.²¹

Historically, influenza vaccine has been recommended to those most likely to have the highest morbidity and mortality; specifically, the elderly and high-risk populations (including all children younger than 5). However, following the 2009 H1N1 epidemic (during which children had a higher rate of hospitalization), seasonal influenza vaccination became the universal recommendation for all children over 6 months of age.²² To fully understand the efficacy of the vaccine, one must understand the virus. Several strains of influenza exist, but the clinically relevant ones include Influenza A and B. The influenza virus is a member of the Orthomyoxoviridae family. Influenza A is further classified based on the expression of two key surface glycoproteins: neuraminidase and hemagglutinin. To date there are 9 and 15 different types of neuraminidase (N) and hemagglutinin (H) proteins, respectively, but only H1, H2, H3, N1, and N2 have been identified in humans. Small point mutations in the DNA coding may occur during viral replication leading to new strains, referred to as a “minor shift.” In addition to antigenic drift (the aforementioned point mutations), pandemics occur as a result of antigenic shift. This occurs when the influenza virus typically found in other animals is identified in humans. All 15 forms of hemagglutinin and 9 forms of neuraminidase are found in birds, and these animal hosts are typically asymptomatic. However, the exchange of genetic material between animal and human strains of influenza result in novel combinations/strains once they infect humans. Every year the Centers for Disease Control and Prevention (CDC) predicts which strains will be most prevalent to facilitate production of a quadrivalent (2A and 2B serotypes) vaccine. Classically, Influenza A (H3N2) has been associated with higher morbidity and mortality, but there have been years where those affected with Influenza B have more significant clinical outcomes.^15,23,24