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Key points:

• There are four kinds of flu: A, B, C and D. They create different strains with names that include details about their type, origin location, year they were first isolated and more.
• Types A and B cause seasonal influenza. Getting your annual vaccine is the best way to protect yourself against severe illness.
• Because the influenza virus changes often, the vaccine gets updated regularly. This is why it’s essential to get your vaccine every year.

If you suddenly come down with fever, chills, body aches, tiredness and sore throat, it may be the influenza virus (flu). When someone coughs or sneezes, flu particles spread through the air. Other people may inhale the particles or touch something where the virus has landed. Once the virus enters your mouth, nose or eyes, it can replicate and cause symptoms.

But did you know there are four types of flu—and many strains of those four types?

Luckily, antiviral medications can help you feel better no matter what kind you have. So read on to learn why these types matter and how they’re named. Plus, how to get online treatment from Dr. B without leaving the couch—so you can get back on your feet!

Types of flu

There are four types of flu viruses. The main ones that affect humans are types A and B.

Flu A: Type A causes most human infections. Compared to other flu viruses, it changes rapidly. It’s the cause of pandemics like the 2009 H1N1 swine flu and the 1918 Spanish flu. During most flu seasons, this type causes about 75% of all infections.

Flu B: The influenza B virus changes more slowly than the A virus, so many adults have some immunity from past vaccination or infection. While it can still cause serious illness in both adults and children, it tends to cause worse illness in kids who haven’t yet been exposed to it, and so causes about 22-44% of flu-related deaths in kids.

Flu C: The influenza C virus can infect people but is usually mild. It doesn’t cause significant epidemics.

Flu D: Type D only affects animals—it’s not a concern for humans.

Which type of flu is the worst?

All types can cause serious illness. Types A and B cause similar proportions of hospitalizations and deaths. But some strains of these types can be more severe than others.

If a new strain emerges that most people are not immune to, it can cause more severe illness. In the 2017-2018 winter season, a type A strain (H3N2) caused a significant number of hospitalizations and deaths in children and older adults.

Why do I need a flu vaccine every year?

When a germ like the influenza virus enters your body, your body recognizes that a foreign invader is present and sends immune cells to investigate. If your body has seen that particular invader before, it triggers special memory cells to respond.

Memory cells develop if you’ve been vaccinated against or have already been infected by that invader.  These memory cells know how the invader works and how to create antibodies to defeat it quickly.

Modern vaccines contain tiny fragments of inactive viruses, which teach our body how to make antibodies to that virus. That way, if you later encounter the virus, your body already knows how to fight it.

Unfortunately, viruses are wired to adapt. (If they didn’t, humans would build enough immunity that viruses could never spread.) The influenza A virus changes particularly quickly. That’s why the flu vaccine has to be updated each year in the United States.

Which type of flu is most common in 2023?

The 2023/2024 flu vaccine is quadrivalent, which means that it protects against four strains. But because the vaccine is developed and produced months before the flu season, it can be tricky to match its target to real-life circumstances.

Luckily, the human immune system is smart. It usually responds effectively if it meets an invader similar to something it’s seen. That means the vaccine still provides some protection even if it doesn’t perfectly match what’s circulating during a given year.

The 2023-2024 flu vaccine contains:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus (in egg-based vaccines) or  A/Wisconsin/67/2022 (H1N1)pdm09-like virus (in cell- or recombinant-based vaccines)
• an A/Darwin/9/2021 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus

Depending on the year, the flu shot is around 40-60% effective at preventing infection. Studies show that it’s most effective against types B and H1N1 A. So far, in 2023, about 75% of all reported cases have been flu A. Of those, about 87% have been H1N1.

How are flu viruses named?

There are a ton of numbers and letters in flu virus names. What do these mean, and why do they matter?

The name of a virus contains a lot of information, including:

• Type of flu (A or B)
• Where it was discovered
• What species it was first discovered in (if other than human)
• The strain number
• The year it was first isolated
• Which protein subtypes are on the virus surface (known as H and N) 
• Whether it’s similar to a variant that caused a pandemic in the past. (For example, pdm09 means pandemic 2009—a.k.a. the 2009 swine flu.)

You’ve probably heard news reports specifying flu strains like H1N1 or H3N2. These letters just refer to the types of proteins on the surface of the virus. Surface proteins are what give viruses their shape and impact how they interact with other cells. The important thing to know is that your body can build an immune response to virus strains of similar shapes—even if they’re not exactly the same.

The 2023 flu vaccine protects against an H1N1 and an H3N2 flu strain. But even if those exact flu strains are not major issues this year, you’ll still likely have some protection against other H1N1 and H3N2 virus strains.

How do I know if I have flu A or flu B?

The only way to know which type you have is to get a nasal swab sample tested by a licensed health provider. Most strains cause similar symptoms and are treated the same way. So for most people, it doesn’t matter which strain you have.

Instead of getting tested, it’s okay to recover at home as long as your symptoms are manageable and you don’t have other health risks. (Like asthma or HIV.)

Where to get flu treatment

For those at high risk for severe illness, prescription antiviral treatment can shorten the virus's duration and make symptoms milder. (Antibiotics only work for bacterial infections, so don’t try taking them for flu!)

But to be effective, you must start these medications within the first two days of your symptoms. Here’s where Dr. B can help. Take a short online assessment, sharing your health history and current symptoms. A licensed medical provider will review your answers within three business hours. If a prescription is appropriate, they’ll send it directly to your local pharmacy.

Dr. B will even help you secure the lowest prescription price at the pharmacy of your choice!

Sources

Baldo, V., et. al (2016). The new pandemic influenza A/(H1N1)pdm09 virus: is it really "new"? Journal of Preventive Medicine and Hygiene.

Centers for Disease Control and Prevention. (2017). Influenza A (H3N2) activity and antiviral treatment of patients with influenza. Centers for Disease Control and Prevention. (2023). Information for the 2023-2024 Flu Season.

Centers for Disease Control and Prevention. (2023). Types of Influenza Viruses.

Centers for Disease Control and Prevention. (2023). Vaccine effectiveness: how well do flu vaccines work?

Centers for Disease Control and Prevention. (2023). Weekly U.S. Influenza Surveillance Report.

No author. (2023). Influenza vaccine for 2023-2024. The Medical Letter.

Nyirenda, M., et al. (2016). Estimating the lineage dynamics of human influenza B viruses. PloS One.

World Health Organization. (2020). How do vaccines work?