The worst season of the year is definitely the flu season. Each year, there's a new vaccine and renewed curiosity about the details of the illness. If you're confused about all the information (and misinformation) about the types of viruses that cause the flu, let's clear that up for you.

There's no difference between influenza A and B in how it feels to have the seasonal flu, but the viruses themselves differ in a few important ways. These essentially boil down to the following:

• Differing genetic structures (and thus, subtypes)
  
• Who or what can contract each type of flu virus
  
• Certain types of antiviral treatments
  
• Mutation rates and pandemic potential
  

Keep reading for more detailed information on the difference between influenza A and B.

What Is Influenza?

Viruses in the Orthomyxoviridae family cause the flu, or influenza. Three types of influenza viruses cause illnesses in humans — A, B and C.

Influenza viruses types A and B cause seasonal flu outbreaks. The yearly flu vaccine protects against both virus types. Influenza type C viruses do not cause seasonal flu but rather a less severe, cold-like illness.

Although they share many similarities, there are structural differences between influenza A and B viruses. They also differ in terms of their capacity to cause a flu pandemic — a worldwide outbreak of a new flu strain.

Types, Subtypes, Strains and Lineage of Influenza A vs B

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Influenza type A and B viruses are genetically similar enough to be included in the same family of viruses. The genetic difference between influenza A and B, however, are significant enough to warrant separating the viruses into two types.

Influenza Type A

Influenza type A viruses are categorized by subtype and strain. Subtype is based on differences in two proteins (designated H and N) on the surface of the virus. The Centers for Disease Control and Prevention (CDC) reports there are 18 H subtypes and 11 N subtypes.

Each influenza type A virus subtype possesses one variant of the H and N proteins, such as H1N1 or H3N2. Influenza A virus subtypes are subdivided into strains based on genetic variations in the H protein of the virus.

Unless you're a virologist, you'll never need to worry about the designation of influenza type A virus strains — but these strains play an important role in why you need a flu shot every year, which we'll get to soon.

Influenza Type B

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Influenza type B viruses are not categorized by subtype but rather by strains and lineages, such as Yamagata and Victoria. The difference in categorization of influenza types A and B is due to differences in how quickly they undergo spontaneous genetic changes. For example, Influenza B symptoms in 2018 may look very different from what they were in 2000.

Read more: 5 Things You Need to Know About the Flu Incubation Period

Who (and What) Contracts Influenza A or B

Another difference between influenza A and B is the range of animal hosts they infect in addition to people.

Wild and domestic birds are the natural hosts for nearly all influenza type A subtypes. Various A subtypes can also infect a variety of mammals including pigs, ferrets, horses, bats — and humans, of course.

Influenza type B viruses infect humans almost exclusively, though they have been rarely found in other mammals such as seals. Influenza type B viruses do not infect birds.

Seasonal Flu Symptoms

Influenza type A and B viruses cause the illness we recognize as the flu with symptoms, including:

• Fever
• Headache
• Dry cough
• Fatigue
• Body aches
• Sore throat
• A runny or stuffy nose
  

So which flu is worse? Well, doctors used to think that people with influenza type B experienced less severe symptoms than those with a type A infection. However, a 2014 study published in Clinical Infectious Diseases and involving more than 26,000 people with seasonal flu found no difference in the severity of the illness based on the influenza virus type.

Read more: Signs & Symptoms of Type A Influenza

Treatments for Influenza A or B

These two main types of flu viruses can also differ in their response to certain treatments. The antiviral medications zanamivir (Relenza), oseltamivir (Tamiflu), peramivir (Rapivab) and baloxavir marboxil (Xofluza) are active against and approved by the U.S. Food and Drug Administration (FDA) for the treatment of influenza types A and B infections.

The antiviral medications rimantadine (Flumadine) and amantadine are FDA approved only for treatment of influenza type A infections because they are not active against type B viruses.

The CDC does not, however, recommend amantadine or rimantadine as first-line treatment for type A influenza because of high levels of resistance to these drugs among circulating strains.

Mutation Rates and Pandemic Potential

The last major difference between influenza A and B is the rate at which these viruses change and evolve (mutation rate), which also impacts how much they can contribute to a flu pandemic. Influenza type A viruses are in a perpetual state of change. Spontaneous changes called mutations occur frequently in their genes.

From one flu season to the next, genetic changes in the circulating influenza type A viruses are extensive enough that your immune system cannot recognize the newly change viruses — even if you had the flu shot or the flu the previous year. This is why you need a flu shot every year; the shot from the previous year cannot protect you against the newly mutated influenza type A viruses.

Influenza type B viruses mutate much more slowly than influenza type A viruses. Whereas type A viruses change significantly from one flu season to the next, influenza B viruses typically change significantly only every few years.

The high mutation rate of influenza type A viruses combined with their broader range of hosts imparts these viruses with pandemic potential that influenza B viruses do not possess. All the influenza pandemics that have occurred in modern times — from the Spanish flu in 1918 to the 2009 novel H1N1 pandemic — have been caused by influenza type A viruses.