What is the typical course and prognosis of influenza (flu)?

Course and Prognosis of Influenza

For most healthy individuals, uncomplicated influenza resolves within 3-7 days, though cough and malaise commonly persist for more than 2 weeks; however, high-risk populations including adults >65 years, infants <1 year, pregnant women, and those with chronic conditions face substantially elevated risks of complications, hospitalization, and death. 1

Typical Disease Course

Incubation and Infectious Period

• The incubation period ranges from 1-4 days, with an average of 2 days 1
• Adults are infectious from the day before symptom onset through approximately 5-6 days after illness begins 1
• Children can be infectious before symptoms appear and remain contagious for up to 10 days after onset 1
• Severely immunocompromised individuals may shed virus for weeks to months 1

Clinical Presentation

• Influenza characteristically presents with abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis 1, 2
• Young children often present atypically with otitis media, nausea, vomiting, and may not report classic influenza symptoms 1
• Infants may initially present with signs mimicking bacterial sepsis, high fever, or febrile seizures (occurring in up to 20% of hospitalized children) 1

Resolution Timeline

• Uncomplicated illness typically resolves after 3-7 days in most individuals 1
• Cough and malaise frequently persist for more than 2 weeks even after other symptoms resolve 1

Prognosis by Risk Category

High-Risk Populations

The following groups face significantly elevated risks of severe outcomes:

• Adults aged >65 years (account for >90% of influenza-related deaths) 1
• Infants aged 0-1 years (hospitalization rates comparable to elderly patients at 200-1,000 per 100,000) 1
• Pregnant women 1
• Individuals with chronic cardiopulmonary conditions 1
• Immunocompromised persons (experience prolonged viral shedding and more severe symptoms) 1

Hospitalization Rates

• Children 0-4 years with high-risk conditions: approximately 500 per 100,000 population 1
• Children 0-4 years without high-risk conditions: approximately 100 per 100,000 population 1
• Adults >65 years: 200 to >1,000 per 100,000 population 1
• Annual U.S. influenza-associated hospitalizations average approximately 114,000, with 57% occurring in persons <65 years 1

Mortality

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

Complications and Their Prognosis

Pulmonary Complications

• Secondary bacterial pneumonia (particularly Staphylococcus aureus, including MRSA strains) carries high mortality 3
• Primary influenza viral pneumonia can occur, each with distinct clinical presentations 1, 3
• Children may develop croup, bronchiolitis, or pneumonia 1

Extrapulmonary Complications

Though uncommon, influenza is associated with serious complications including: 1

• Encephalopathy and transverse myelitis
• Myositis, myocarditis, and pericarditis
• Reye syndrome (particularly in children)
• Exacerbation of underlying cardiac or pulmonary disease

Pediatric-Specific Outcomes

• Mortality rate for hospitalized children with laboratory-confirmed influenza: 0.6% 1
• Overall mortality for all laboratory-confirmed influenza in children: 0.1% 1
• Most pediatric deaths occur in children without known high-risk conditions, though deaths are more frequent among those with risk factors 1

Critical Clinical Pitfalls

Important diagnostic limitation: 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 1, 4

Do not rely on "sudden onset" as a diagnostic criterion - positive predictive value ranges from only 30% in elderly patients to 79-88% in healthy adults 4