Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. While many people associate the flu with a few days of fever and discomfort, the severity of the disease varies widely depending on the specific strain of the virus circulating. The seasonal flu is responsible for millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths each year globally. The differences in potential severity are determined by how scientists classify and compare the various strains, which informs public health responses and vaccine development.
The Major Categories of Influenza
The influenza virus is divided into four main types: A, B, C, and D. Only Types A and B are the primary causes of the annual seasonal epidemics in humans. Type A is the most diverse and is found in both humans and animals, which gives it the potential to cause global pandemics. Type B viruses circulate almost exclusively among humans and can still cause significant seasonal outbreaks, but they do not typically cause pandemics.
Type C influenza infections generally cause a very mild respiratory illness, often with symptoms similar to the common cold, and are not associated with large-scale epidemics. Influenza Type D viruses are primarily found in cattle and have not been known to infect people. Because Type A and Type B viruses are responsible for the vast majority of serious human disease, they are the focus of global surveillance and annual vaccine efforts.
Comparing the Severity of Influenza A Subtypes
Influenza Type A viruses are further categorized into subtypes based on the combination of two surface proteins, hemagglutinin (H) and neuraminidase (N). The two main Type A subtypes that circulate in humans during flu season are A(H1N1) and A(H3N2).
Historically, the A(H3N2) subtype is associated with more severe seasons, often leading to higher rates of hospitalization and death. This strain tends to cause more severe illness in older adults, who may have diminished immune responses to its rapid evolution, and in very young children.
Seasons dominated by A(H3N2) typically result in a higher overall disease burden compared to seasons dominated by A(H1N1). However, A(H1N1) can be disproportionately severe in younger, otherwise healthy adults, as seen with the pandemic strain that emerged in 2009. While A(H3N2) drives higher overall mortality, A(H1N1) has been linked to a higher likelihood of severe in-hospital outcomes, including increased Intensive Care Unit admissions. Influenza B, though generally considered less severe than Type A, can still cause significant illness and is particularly problematic for children.
Why Certain Strains Cause Greater Illness
The variation in a strain’s severity and its ability to spread is directly linked to the way the virus changes over time. One mechanism is called antigenic drift, which involves small, continuous changes, or mutations, in the genes of the circulating influenza viruses. These minor changes accumulate over time, slightly altering the surface proteins so that a person’s existing immunity or a previous year’s vaccine may no longer recognize the virus effectively. Antigenic drift occurs in both Type A and Type B viruses and is the reason the flu vaccine must be updated annually.
A more dramatic change is called antigenic shift, which only occurs with Type A viruses and is the cause of pandemics. Shift is an abrupt, major change that results in a completely new hemagglutinin or neuraminidase protein combination. This can happen if an animal influenza virus gains the ability to infect humans, or if two different influenza viruses swap genetic material. When an antigenic shift occurs, the vast majority of the human population has little to no immunity against the novel strain, which allows the virus to spread rapidly and cause widespread, severe illness.
How Researchers Track and Predict Severe Strains
A global surveillance network is employed by organizations like the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) to monitor circulating influenza strains year-round. Laboratories across the globe continuously collect and analyze influenza samples to track the genetic and antigenic properties of the viruses. This information is used to identify new variants that are emerging and spreading.
The data gathered from this surveillance is used in predictive models that forecast which strains are most likely to be dominant and cause the most illness in the upcoming flu season. These models analyze the rate of genetic change and the frequency of different strains to determine which viruses have an evolutionary advantage. This prediction process, which must occur months in advance, directly informs the selection of the components for the annual influenza vaccine to maximize its effectiveness against the anticipated severe strains.