The term “A&B strain” of the flu describes the two types of influenza virus responsible for most seasonal flu: Influenza A and Influenza B. While both are highly contagious and cause similar respiratory symptoms, they are biologically distinct viruses. They represent the primary targets for annual vaccination efforts and antiviral treatments.
Key Differences Between Influenza A and B
Influenza A viruses are categorized into subtypes based on two proteins on the virus’s surface: hemagglutinin (H) and neuraminidase (N). This gives rise to familiar names like H1N1. A characteristic of Influenza A is its ability to infect a wide range of animal species, including birds and pigs. This cross-species capability allows the virus to undergo significant genetic changes, leading to new and unpredictable strains.
This capacity for major genetic shifts makes Influenza A the only type responsible for flu pandemics. In contrast, Influenza B viruses primarily infect humans and mutate at a much slower rate. Instead of subtypes, Influenza B is classified into two main genetic lineages: B/Yamagata and B/Victoria. This slower rate of change means that Influenza B does not cause pandemics.
Symptoms and Severity
The symptoms of Influenza A and B are nearly identical, making it impossible to differentiate between them without a laboratory test. The illness typically begins suddenly with symptoms such as:
- Fever or chills
- Cough and sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headaches and fatigue
In some cases, particularly in children, gastrointestinal symptoms like vomiting and diarrhea can also occur.
While both types can lead to severe illness, hospitalization, and complications, Influenza A is more frequently associated with widespread and severe seasonal epidemics. Some studies suggest that Influenza A is associated with higher rates of pneumonia, while Influenza B can be particularly severe in young children. Despite these tendencies, both viruses are capable of causing significant disease in vulnerable populations, including the elderly, young children, and individuals with underlying health conditions.
Prevention and Treatment
The most effective prevention against both Influenza A and B is the annual flu vaccine. Each year, scientists predict which strains are most likely to circulate, and vaccines are formulated to protect against them. Quadrivalent flu vaccines, for example, protect against two Influenza A viruses and both Influenza B lineages (B/Yamagata and B/Victoria).
If an infection occurs, antiviral medications can treat illnesses caused by both Influenza A and B. Drugs such as oseltamivir phosphate are effective against both types and prevent the virus from spreading inside the body. For these treatments to be most effective, they should be started within 48 hours of symptoms appearing. Early treatment can lessen the severity of symptoms and may reduce the illness duration by about a day.