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We're kicking it back to kindergarten to learn the ABCs of flu. From influenza A, B, C, and D to everything else in between, find out what the different flu strains mean for you and your family.


October 7, 2019

Remember that swine flu pandemic in the United States in 2009? Yeah, we're talking about the one that might've gotten you a week or two off school or kept you out of the office for a while. No matter where you lived at the time, it was likely big news. That's because it was a brand new virus and, unlike seasonal flu, which comes around every year, most people had little or no immunity to it. In fact, the Centers for Disease Control and Prevention (CDC) estimates that nearly 61 million people were infected with the swine flu during that very pandemic.1

Have you ever stopped and wondered what the heck "swine flu" really means? Is it similar to the seasonal flu? And can you really catch it from a pig? And, while we're at it, what about those other strains of flu, like bird flu and influenza A, B, C, and D? Well, we're here to give you a full rundown of the different strains of the flu, how they get their names, and what they mean for you.

Lesson #1: Flu Strain Basics

Okay, we know we're not all biology majors, so we're going to take this slow and start from the very beginning. One of the most important things for you to know is that there are four types of influenza viruses: A, B, C, and D. Influenza A and B viruses are the ones we know really well – in fact, they're the ones that cause those almost-annual seasonal epidemics in the United States. Influenza C typically causes a mild respiratory illness and doesn't typically cause epidemics. Finally, influenza D mostly affects cattle (yes, as in "moo" and other similar animals) and, at least for now, is not known to infect people. Anyway, more on the specifics strains later.

The second most important thing for you to understand is how these viruses get their names. Influenza A viruses are all characterized based on two proteins on the outer shell (or, if you want to get fancy, the "envelop") of the virus: hemagglutinin (the H in H1N1) and neuraminidase (the N in H1N1). Say those names ten times fast! These proteins both must be present in order for the virus to replicate, or reproduce. Hemagglutinin helps the virus attach to the cells in your body, and neuraminidase allows the virus to be released from the host cell once it has replicated – ultimately spreading the infection.

a woman sick with the flu blowing her nose into a tissue while outside

Phew – still with us? Okay, good.

What about those numbers, you ask? Well, they're determined by the subtype, or secondary type, of hemagglutinin and neuraminidase proteins; currently, there are 18 known hemagglutinin subtypes (H1 to H18) and 11 known neuraminidase subtypes (N1 to N11), according to the CDC.2

Influenza B viruses are not designated by those same subtypes. Instead, they're broken down into lineages, or where each virus was first isolated. In the case of influenza B, those lineages are Yamagata and Victoria. We won't go too far into that, though, but it's important to know that the quadrivalent flu vaccine helps protect against both of these lineages each flu season.

Lesson #2: Influenza A Symptoms

Now that you're ready to ace your next biology exam on flu virus nomenclature (or maybe just win a few extra brownie points with your teacher), we're going to jump into Lesson #2: Influenza A.

Influenza A is typically responsible for those seasonal influenza epidemics that come around almost every winter. In the US, cases of seasonal flu typically spike during a few specific months of the year starting in September or October and running all the way through March or April (and even sometimes into May!) This strain of the flu is also typically the most serious, and often has more severe symptoms than other strains, including coughing, sneezing, sore throat, fever, exhaustion, and chills and body aches.

This strain of the flu is most commonly found in humans and animals. In fact, remember that swine flu pandemic we were talking about at the beginning of this blog? Well, that virus (H1N1) is technically an influenza A virus, but it's a little bit more complicated than that. That particular strain was so severe because it was a combination of swine, human, and bird flu viruses – talk about a triple threat! The strain mutated so that it was no longer the same influenza A virus that our bodies are protected by through the annual flu vaccine.

Flu Mutation? Did You Really Just Say That? 

Why, yes, we did. But we're not talking about the type of mutation you might remember from your childhood comics. What we mean is that flu viruses are always changing, evolving, and mutating. That's why the flu shot may be more effective some years than others. When developing the flu shot each year, scientists predict what strains will be predominant during the upcoming season, and create the flu vaccine based off of those strains. However, sometimes, flu strains change through antigenic drift or antigenic shift. Let's talk about that for a minute.

Antigenic drift is a process in which small changes to the virus's genes are made over time as it replicates, or reproduces. These changes are usually small and, as a result, the new viruses are closely related to the original virus. However, over time, these changes can certainly build upon each other and eventually result in a completely foreign virus that the body might not recognize.

Antigenic shift on the other hand is a – you guessed it – big ole' shift, or change, to influenza A viruses. Such a big change, in fact, that it results in new hemagglutinin or neuraminidase proteins, meaning that the very subtype of the virus has changed completely.

Which brings us back to the 2009 swine flu pandemic. In the spring of 2009, an antigenic shift occurred to the circulating flu strain – and a completely new virus emerged to which people had little to no protection against. It sounds a little scary, we know – but unlike antigenic drift, which happens all the time, antigenic shift happens only occasionally, so pandemics like the 2009 swine flu only come around once in a while. And it turns out scientists are getting even better at predicting and preparing for them (more on that in our Future of Flu blog).

Lesson #3: Influenza B Symptoms

Also responsible for that pesky seasonal flu, influenza B has very similar symptoms to the influenza A virus, including coughing, sneezing, fatigue, fever, and body aches. Unlike influenza A, though, influenza B is only spread from human to human, and cannot infect animals. While it's a little bit less common, influenza B can be just as severe and contagious as influenza A.

a woman sick with the flu sitting on a couch and holding a tissue against her nose

You may remember that the 2017-2018 flu season was one of the most severe seasons we've seen in several years. The reason behind that, scientists believe, all goes back to the circulating strains that season (how on topic!) During a typical season, one strain – either influenza A or influenza B – usually dominates. But during the 2017-2018 season, both influenza A and influenza B circulated at the same time. As you can imagine, that's a heck of a lot of flu to go around (and all the more reason to wash your hands!)

Lesson #4: Influenza C Symptoms

Ah, sweet, sweet influenza C. Influenza C is the mildest of all of the strains. In fact, it typically only causes minor respiratory symptoms. Far less is known about influenza C because it's much rarer and not easily isolated, so scientists aren't able to study it in depth.

Plus, influenza C isn't capable of undergoing antigenic shift (remember, that's the big one that can cause strains to mutate quickly and abruptly), so it's not a significant concern for humans.

Lesson #5: Bird and Swine Flu Symptoms

You probably feel pretty smart right now, huh, with all that that new flu strain knowledge rolling around in that big brain of yours. But we're not done yet. You're probably wondering when we're going to talk about bird, or avian flu and swine flu. Well, buckle up.

We already talked a little bit about swine flu, which was given that name because the virus spread easily among pigs (and rarely spread among humans). The symptoms of swine flu in humans are similar to that of the seasonal flu, but gastrointestinal issues like vomiting and diarrhea are more common.

Like swine flu, bird flu doesn't typically infect humans. However, when an infected bird sheds the virus through saliva, mucus, or feces, and the virus gets into a person's eyes, nose, or mouth, it can spread to humans. That's why it's very important to protect yourself when working closely with birds; always wear gloves and a mask. Because symptoms of bird flu and swine flu can look similar to seasonal flu, lab testing is needed to confirm a diagnosis.  

Right now, bird flu rarely spread from person-to-person. But should the strain mutate, it's certainly possible that bird flu could become a bigger public health concern in the future. That's why scientists are studying it and monitoring for bird and human infection all the time.

The good news is that the CDC believes the risk to the health of people in the United States as a result of bird or swine flu is currently low.3 But that doesn't mean we should get lazy with our hygienic practices. Slow the spread of germs – especially during flu season – by washing your hands frequently, covering your mouth and nose when you cough or sneeze, and staying home when you're sick. And, of course, always get your annual flu shot.

MedExpress Pro Tip: If you've ever found a dead bird in your yard and you're not sure what to do with it, call your local authorities. Some state and local agencies have different policies, and may want to come collect the bird for testing. Never, ever handle a dead bird (or any undomesticated bird) with your bare hands. Always use gloves and, if possible, wear a mask.

Well, that about wraps up today's lesson. But if there's one thing we want you to take away from all this, it's that it's important to get your annual flu shot so you and your family can help get protected against the strains that can really make you sick. And if you do come down with the flu this season, visit a healthcare professional right away – there are certain treatments that can help reduce the severity of symptoms and possibly get you back on your feet faster.



1 Centers for Disease Control and Prevention: CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations, and Deaths in the United States. Accessed August 15, 2019.

2 Centers for Disease Control and Prevention: Types of Influenza Viruses. Accessed August 15, 2019.

3 Centers for Disease Control and Prevention: Avian Influenza Current Situation Summary. Accessed August 19, 2019.

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