The flu, or influenza, is a contagious respiratory illness caused by viruses that infect the nose, throat, and sometimes the lungs. This illness appears every year during the colder months. It is caused by different types of influenza viruses, with Type A and Type B being the most common to affect humans.
Structural and Epidemiological Distinctions
Influenza A is classified based on two surface proteins, hemagglutinin (H) and neuraminidase (N). This leads to numerous subtypes circulating in humans, such as H1N1 and H3N2. This extensive classification allows for a wide array of potential strains, contributing to the virus’s adaptability.
Influenza B is not categorized by H and N subtypes but is divided into two distinct genetic lineages: Victoria and Yamagata. Influenza A can infect multiple species, including humans, birds, and pigs, allowing for genetic mixing and reassortment, known as antigenic shift. This major, abrupt change creates entirely new strains to which humans have little immunity, giving Influenza A the potential to cause pandemics.
Both Type A and Type B viruses undergo antigenic drift, a continuous, minor change in surface proteins. However, Influenza B almost exclusively infects humans, lacking the animal reservoir that drives the major genetic shifts seen in Type A. This limited host range means that Influenza B tends to cause seasonal epidemics but cannot cause a global pandemic.
Comparing Illness Severity and Risk
The clinical presentation of both flu types is similar, including fever, body aches, and fatigue. Influenza A is associated with more severe seasonal outbreaks, higher rates of hospitalization, and greater mortality across all age groups. This severity is due to the rapid, extensive variability of the Type A virus, which makes it harder for the immune system to maintain long-term protection.
While Flu A is the more common cause of severe illness in adults, Flu B can be particularly severe in children. In some seasons, Flu B has been linked to a higher risk of complications, such as seizures or gastrointestinal issues, in pediatric patients.
Only Influenza A possesses the biological mechanism and host range necessary to cause human pandemics, making it the primary focus of global surveillance efforts. Type A viruses can jump from animal hosts to humans, bypassing existing population immunity and spreading rapidly worldwide.
Implications for Vaccination and Treatment
Current influenza vaccines are designed to protect against both types. They typically include two strains of Influenza A and either one or two lineages of Influenza B. Quadrivalent vaccines, for instance, protect against the two main Influenza A strains and both the Victoria and Yamagata lineages of Influenza B.
The selection process involves complex global surveillance to predict which strains of Type A and which lineages of Type B will circulate. While Type B viruses are more stable, the high mutation rate and constant shifting of Type A strains make predicting the dominant Flu A components a continuous challenge.
Antiviral medications, such as neuraminidase inhibitors, are effective against both Influenza A and Influenza B viruses. These treatments work by inhibiting the virus’s ability to spread between cells, shortening the duration and severity of the illness. Common antivirals are prescribed for both types to reduce the risk of serious complications, particularly in high-risk patients.