Influenza is a contagious respiratory illness caused by influenza viruses. These viruses infect the respiratory tract, causing symptoms like fever, cough, sore throat, muscle pain, and fatigue. While many people recover from the flu within a week or two, it can lead to serious complications, particularly for vulnerable populations. There are different types of influenza viruses, with types A and B being the primary causes of seasonal epidemics.
Understanding Influenza A
Influenza A viruses infect a wide range of animal species, including humans, birds, and pigs. Wild aquatic birds are considered the natural reservoir for these viruses. The ability of Influenza A to infect multiple hosts contributes to its potential for generating new strains and causing widespread outbreaks.
The high variability of Influenza A viruses stems from two main processes: antigenic drift and antigenic shift. Antigenic drift involves small, gradual mutations in the virus’s surface proteins, leading to new strains that the immune system may not recognize as effectively. Antigenic shift is a more abrupt, major change, occurring when two different influenza A viruses swap genetic material, creating a novel subtype to which humans have little or no immunity. This capacity for significant genetic change makes Influenza A the only type known to cause global pandemics. Current subtypes circulating in humans include H1N1 and H3N2.
Understanding Influenza B
Influenza B viruses primarily affect humans. While occasionally found in other mammals, Influenza B circulation is largely limited to humans. This restricted host range means that Influenza B viruses do not typically cause global pandemics.
Influenza B viruses evolve through antigenic drift, similar to Influenza A. However, the rate of genetic change in Influenza B is slower than that of Influenza A. Influenza B viruses are classified into two main genetic lineages: B/Victoria and B/Yamagata. Both lineages have historically co-circulated in human populations, though the B/Yamagata lineage may have become extinct since March 2020.
Comparing Severity: Which is Worse?
Historically, Influenza A has been considered to cause more severe illness, larger outbreaks, and to be the sole cause of pandemics. This is largely attributed to its capacity for antigenic shift and ability to infect various animal species, allowing novel strains to emerge against which humans have limited immunity. For instance, the 2009 H1N1 pandemic was caused by an Influenza A virus.
While Influenza A is associated with greater overall severity and pandemic potential, individual illness severity can vary significantly for both types. Studies concerning hospitalized adults show that Influenza B can cause equally severe disease outcomes, including comparable rates of ICU admission and death. Influenza B infections affect children and young adults more frequently. In children, Influenza B has been associated with a higher risk for mortality.
Implications for Public Health and Prevention
Understanding the distinctions between Influenza A and B is important for public health initiatives. Both Influenza A and B viruses contribute to seasonal flu epidemics, so annual flu vaccines protect against circulating strains of both types. Seasonal flu vaccines include components targeting one Influenza A(H1N1) virus, one Influenza A(H3N2) virus, and one Influenza B/Victoria lineage virus.
Ongoing surveillance is important to track the circulating strains of both Influenza A and B to ensure vaccine effectiveness. Antiviral medications are effective against both Influenza A and B viruses. These medications can reduce the severity and duration of illness, particularly when started within 48 hours of symptom onset. Regardless of which type of influenza is circulating, prevention through annual vaccination and early treatment when indicated remain important strategies for reducing the impact of the flu.