Which type of influenza, Influenza A or Influenza B, is more severe, particularly in vulnerable populations such as the elderly, young children, and those with underlying health conditions?

Which Influenza Type is More Severe?

Influenza B has significantly higher mortality rates than Influenza A in hospitalized patients, particularly in children, though the overall clinical severity between the two types is more nuanced than previously thought. 1

Mortality Data: The Critical Distinction

The most important finding from recent pediatric guidelines is that 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. 1 This represents a paradigm shift from conventional thinking that influenza A is universally more severe.

Pediatric Death Statistics (2019-2020 Season)

During the 2019-2020 influenza season, the mortality pattern was striking: 1

• 122 pediatric deaths were associated with influenza B viruses
• 77 pediatric deaths were associated with influenza A viruses
• This occurred despite influenza A being more prevalent overall in circulation

This nearly 60% higher death count from influenza B occurred even though influenza A(H1N1)pdm09 was the predominant circulating strain for most of the season. 1

Clinical Severity: A More Complex Picture

However, when examining broader clinical parameters beyond mortality, the picture becomes more balanced:

Hospitalization and Clinical Features

A comprehensive 14-year study in hospitalized children found no significant differences in clinical features, outcomes, intensive care unit treatment, or length of stay between influenza A and B infections when analyzed across all age groups. 2 This included: 2

• Similar rates of blood culture collection (36.2% for A vs 34.8% for B)
• Similar rates of lumbar puncture (5.7% for A vs 9.8% for B)
• Comparable overall clinical presentations and management requirements

Hospitalization Rates

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—higher than rates seen during the 2009 H1N1 pandemic and exceeding rates in adults 50-64 years old. 1 However, these high rates occurred during a season with mixed A and B circulation, making type-specific attribution complex.

Age-Specific Considerations

Young Children (0-4 Years)

Children under 5 years, particularly those under 2 years, face the highest risk regardless of influenza type. 3 The 2019-2020 season saw: 1

• 48.6% of hospitalized children had no underlying conditions
• Most common comorbidities: asthma/reactive airway disease (22.1%), neurologic disorders (17.5%), obesity (12%)

Older Adults

While the evidence focuses heavily on pediatric populations, older adults (>65 years) account for >90% of influenza-related deaths overall, with death rates ranging from 30 to >150 per 100,000 during epidemics. 4 The type-specific mortality differences in this population are less well-characterized in the provided evidence.

Clinical Implications and Pitfalls

Critical Takeaway for Practice

Do not assume influenza B is "milder" based on historical teaching. 1 When managing hospitalized patients, particularly children, influenza B should be treated with equal or greater concern regarding mortality risk.

Vaccination Considerations

The higher mortality associated with influenza B underscores the importance of quadrivalent vaccines containing both B lineages rather than trivalent formulations. 2 Vaccine effectiveness against death in children is 65% overall and 51% in those with underlying conditions. 3

Diagnostic Approach

Respiratory illnesses caused by influenza cannot be reliably distinguished from other pathogens based on symptoms alone, with clinical definitions showing only 63-78% sensitivity and 55-71% specificity compared to viral culture. 4 Laboratory confirmation is essential for accurate type identification and epidemiological tracking.

High-Risk Population Identification

Regardless of influenza type, prioritize aggressive management in: 4, 3

• Children 0-4 years (especially <2 years)
• Adults >65 years
• Pregnant women
• Individuals with chronic cardiopulmonary conditions
• Immunocompromised patients

Historically, up to 80% of pediatric influenza deaths occur in unvaccinated children, emphasizing prevention over treatment. 1, 3