Influenza is a respiratory illness caused by viruses that circulate seasonally. While people commonly refer to “the flu” as a single disease, the infection is caused by different types of influenza viruses. Four main types exist—A, B, C, and D—but only Type A and Type B are responsible for the annual seasonal epidemics. Although both types cause similar symptoms, they possess fundamental biological differences that alter their behavior, public health impact, and preventative measures.
Fundamental Biological Distinctions
Influenza A viruses are classified based on two surface proteins: Hemagglutinin (H) and Neuraminidase (N). Numerous combinations of these proteins exist, allowing for a wide range of subtypes such as H1N1 and H3N2. Type A viruses constantly change and recombine genetic material, partly due to their wide host range, which includes humans, birds, and various mammals.
In contrast, Influenza B viruses are not categorized into H and N subtypes. Instead, they are classified into two distinct genetic lineages: B/Victoria and B/Yamagata. This structural difference means that Type B viruses evolve more slowly than Type A viruses, typically undergoing a gradual change called antigenic drift. Furthermore, Type B viruses have a much more limited host range, circulating almost exclusively among humans.
Differences in Seasonal Impact and Severity
The unique genetic plasticity of Influenza A gives it the potential to cause global outbreaks, or pandemics. This occurs because the virus can acquire entirely new combinations of H and N proteins, often by jumping from an animal reservoir to humans. This creates a novel strain to which the human population has little existing immunity. Influenza B viruses lack this mechanism, meaning they only cause seasonal epidemics, not pandemics.
In a typical flu season, Type A viruses are often detected earlier and tend to be the dominant strain, driving the majority of cases and hospitalizations. Type B activity often appears later, sometimes causing a second wave of illness toward the end of the season. While Type A is associated with more severe outcomes across all age groups, Type B infections can disproportionately affect children and young adults.
It is a misconception that Type B is always milder than Type A. Studies among hospitalized adults have shown that Type B infection can be just as severe as Type A, with comparable rates of intensive care unit admissions and mortality. The overall burden of disease, however, is usually higher with Type A simply because it is more prevalent and accounts for a larger percentage of total infections each year.
Vaccine Targeting and Protection
The annual seasonal influenza vaccine is designed to provide protection against the strains predicted to circulate during the upcoming season. Current vaccines are formulated to protect against two Type A strains (typically H1N1 and H3N2) and one or two Type B lineages. Historically, quadrivalent vaccines protected against both the B/Victoria and B/Yamagata lineages.
The composition of the vaccine is dynamic and is updated yearly based on global surveillance data. Due to the lack of confirmed detection of the B/Yamagata lineage since early 2020, health authorities have recommended removing this component. Consequently, many seasonal vaccines are now transitioning to a trivalent formulation, which targets the two Type A components and only the B/Victoria lineage. Common antiviral medications are effective against both Influenza A and Influenza B viruses.