Influenza A and B Outbreaks: Why the Flu Season Comes in Waves

David S. Klein, MD FACA FACPM
Jan 15
3 min read

A Familiar but Important Pattern

Seasonal Influenza Activity
Chart illustrating the typical seasonal pattern of influenza, with an early Influenza A peak followed by a later Influenza B surge. — Seasonal Influenza Outbreaks are Predictable

Each winter, influenza follows a somewhat predictable epidemiologic pattern. This season has been no exception. Influenza A emerged early and aggressively, driving the initial surge in hospitalizations, urgent care visits, and missed work and school¹². More recently, Influenza B has begun to circulate more widely, prolonging the overall flu season and catching many patients off guard³⁴.

Understanding the differences between these two strains helps explain why flu symptoms may persist in the community even after the initial wave appears to subside.

Influenza A: The Early and More Severe Wave

Influenza A viruses are responsible for most seasonal epidemics and pandemics. They mutate rapidly and are often associated with more severe disease, particularly in older adults, young children, and those with chronic medical conditions⁵⁶.

This season’s Influenza A outbreak was characterized by:

High early transmission rates
Increased emergency department utilization
Significant systemic symptoms (high fever, myalgias, profound fatigue)¹⁷

Influenza A is also more likely to cause complications, including pneumonia, cardiac stress, and worsening of underlying pulmonary disease⁸⁹.

Influenza B: The Later, Lingering Threat

As Influenza A activity begins to decline, Influenza B often rises, particularly later in the flu season³¹⁰. While sometimes perceived as “milder,” Influenza B can still cause significant illness, especially in children, adolescents, and older adults¹¹¹².

Notably:

Influenza B circulates almost exclusively in humans
It tends to spread later in the season
It can prolong community-wide illness even when people believe “flu season is over”⁴¹³

Patients may assume they have a new respiratory virus or a “bad cold,” when in fact they are experiencing a second, distinct influenza infection.

Influenza A vs Influenza B Comparison Diagram comparing Influenza A and Influenza B, including timing of outbreaks, transmission patterns, and typical clinical severity.

Why This Matters Clinically

From a physician’s perspective, the sequential appearance of Influenza A followed by Influenza B explains why:

Flu activity seems prolonged
Patients may become ill twice in one season
Ongoing vigilance remains necessary even late in winter¹⁴

Vaccination, early testing, and timely antiviral treatment remain essential tools, particularly for high-risk individuals¹⁵.

Practical Takeaways for Patients

Influenza A typically strikes earlier and harder
Influenza B often extends the season
Fever, body aches, cough, and fatigue should still prompt evaluation
Antiviral treatment is most effective when started early
Preventive measures remain important even late in the season

Influenza is not a single event—it is a dynamic, evolving outbreak that unfolds in phases.

Summary

This year’s influenza season has followed a classic but clinically important pattern: an early Influenza A surge followed by a later Influenza B wave. Recognizing this progression helps patients understand why flu activity persists and why ongoing awareness, testing, and prevention remain critical well beyond the initial outbreak.

References

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