Influenza Course and Prognosis

Typical Disease Course

The incubation period ranges from 1-4 days, with an average of 2 days, according to the Centers for Disease Control and Prevention (CDC) 1, 2, 3
Adults are infectious from the day before symptom onset through approximately 5-6 days after illness begins, as reported by the CDC 1, 2, 3
Children can be infectious before symptoms appear and remain contagious for up to 10 days after onset, according to the American Academy of Pediatrics 2, 4
Severely immunocompromised individuals may shed virus for weeks to months, as noted by the CDC 4, 3
Influenza characteristically presents with abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis, as described by the CDC 1, 2
Young children often present atypically with otitis media, nausea, vomiting, and may not report classic influenza symptoms, according to the American Academy of Pediatrics 2, 4
Infants may initially present with signs mimicking bacterial sepsis, high fever, or febrile seizures, as reported by the American Academy of Pediatrics 2, 4
Uncomplicated illness typically resolves after 3-7 days in most individuals, as noted by the American Academy of Pediatrics 2, 4
Cough and malaise frequently persist for more than 2 weeks even after other symptoms resolve, according to the CDC 1, 2, 3

The following groups face significantly elevated risks of severe outcomes: adults aged >65 years, infants aged 0-1 years, pregnant women, and individuals with chronic cardiopulmonary conditions, as reported by the CDC 1, 3, 2, 4
Adults aged >65 years account for >90% of influenza-related deaths, according to the CDC 1, 3
Infants aged 0-1 years have hospitalization rates comparable to elderly patients at 200-1,000 per 100,000, as noted by the CDC 1, 3, 5
Children 0-4 years with high-risk conditions have approximately 500 per 100,000 population hospitalization rates, according to the CDC 1, 3, 5
Children 0-4 years without high-risk conditions have approximately 100 per 100,000 population hospitalization rates, as reported by the CDC 1, 3
Adults >65 years have 200 to >1,000 per 100,000 population hospitalization rates, according to the CDC 1, 3, 5
Annual U.S. influenza-associated hospitalizations average approximately 114,000, with 57% occurring in persons <65 years, as noted by the CDC 1, 3

During influenza epidemics, estimated death rates range from 30 to >150 deaths per 100,000 persons aged >65 years, according to the CDC 1, 3
Pediatric deaths range from 44-68 annually in recent seasons, with 94% of deaths occurring in unvaccinated children, as reported by the American Academy of Pediatrics 2, 4
More than 20,000 influenza-associated deaths occur during severe epidemic years in the United States, according to the CDC 1, 3

Children may develop croup, bronchiolitis, or pneumonia, as noted by the American Academy of Pediatrics 2, 4
Influenza is associated with serious complications including encephalopathy, transverse myelitis, myositis, myocarditis, pericarditis, and Reye syndrome, according to the CDC 1, 2, 3
Exacerbation of underlying cardiac or pulmonary disease can occur, as reported by the CDC 1

Mortality rate for hospitalized children with laboratory-confirmed influenza: 0.6%, according to the American Academy of Pediatrics 2
Overall mortality for all laboratory-confirmed influenza in children: 0.1%, as noted by the American Academy of Pediatrics 2
Most pediatric deaths occur in children without known high-risk conditions, though deaths are more frequent among those with risk factors, according to the American Academy of Pediatrics 2, 4

Respiratory illnesses caused by influenza are difficult to distinguish from other respiratory pathogens based on symptoms alone, with clinical definitions showing sensitivity of only 63-78% and specificity of 55-71% compared to viral culture, as reported by the CDC 1, 3, 6

The American Academy of Pediatrics notes that influenza B has significantly higher mortality rates than influenza A in hospitalized patients, particularly in children 7, 8
The odds of mortality were significantly greater with influenza B than with influenza A, and this difference was not entirely explained by underlying health conditions 7, 8

During the 2019-2020 influenza season, 122 pediatric deaths were associated with influenza B viruses, and 77 pediatric deaths were associated with influenza A viruses 7

The 2019-2020 season demonstrated record-breaking hospitalization rates in young children, with cumulative rates of 92.3 per 100,000 in children 0-4 years old 9

Children under 5 years, particularly those under 2 years, face the highest risk regardless of influenza type, with 48.6% of hospitalized children having no underlying conditions 9
The most common comorbidities in hospitalized children were asthma/reactive airway disease (22.1%), neurologic disorders (17.5%), and obesity (12%) 7

The American Academy of Pediatrics recommends not assuming influenza B is "milder" based on historical teaching, and treating it with equal or greater concern regarding mortality risk in hospitalized patients, particularly children 7, 8
Historically, up to 80% of pediatric influenza deaths occur in unvaccinated children, emphasizing prevention over treatment 10