Influenza A vs. Influenza B: Which Flu Virus Is Worse?

Updated: Feb. 07, 2023

Here's what you need to know about the similarities and differences between influenza A and B, including whether their symptoms vary and how the flu shot can help prevent both.

What to do about flu

Covid-19 has killed an astounding 300,000 Americans, but don’t forget that influenza, also known as the flu, also typically kills tens of thousands of people in the U.S. each year. It’s already flu season in the Northern Hemisphere, so flu viruses are  circulating.  (Learn the difference between Covid-19 symptoms vs. flu symptoms.)

It’s important to know that a simple flu shot could reduce your risk of getting seriously ill with the flu, and you should get one right away if you haven’t already. But you should also know about the different types of flu and how they may—or may not—affect you.

Here’s what you need to know about each type, including their similarities, differences, and how the vaccine addresses both.

What are the different types of influenza virus?

There are four different types of influenza virus: A, B, C, and D. Influenza C usually causes only mild illness while D mostly affects animals, especially cattle.

The influenza A and B versions that can infect humans have different proteins and they behave a little differently. Influenza A tends to mutate rapidly while influenza B is stable from year to year, meaning it doesn’t change much, explains Joshua Septimus, MD associate professor of clinical medicine at Houston Methodist.

Which flu virus is mostly likely to cause a pandemic?

Influenza A is really the only flu virus type that can cause a pandemic, says Beth Oller, MD, a practicing family physician in Stockton, Kansas. A flu pandemic is when a new type of influenza virus emerges—that most people aren’t immune to—and spreads around the world.

That’s partly because influenza A infects not just humans but also birds (avian influenza),  pigs, and other animals (even whales). “Because of these wide number of different carriers, influenza A has a tendency to change much more quickly than flu B,” she explains.

Some viruses eventually mutate to the point where they can jump from animals to humans and then start spreading from human to human. It’s these brand-new viruses that our immune systems have never encountered before that have the potential to turn into pandemics. That is exactly what happened with the 2009 H1N1 swine flu and the Spanish flu of 1918 pandemics.

Influenza A subtypes

Influenza A (but not B) also has subtypes labeled H and N. These refer to hemagglutinin (H) and neuraminidase ( N), which are proteins on the surface of the virus. There are 18 different HA subtypes and 11 different NA subtypes. This makes for 198 different subtype combinations. However, researchers have only documented 131, which can circulate at different times.

These surface proteins are what your immune system looks at when the virus first invades, explains Dr. Oller. If your body recognizes these antigens, your body has already made the antibodies you need to seek out and destroy any familiar invaders. If the H and N proteins are totally new, your immune system needs to recognize these and make new antibodies.

Does the flu vaccine cover both A and B?

Yes—every year. Because the flu virus strains that circulate in the population change every year (especially A), manufacturers have to create new vaccines each season. Most vaccines today are “quadrivalent,” meaning they cover four strains, says Dr. Oller.

These are almost always two flu A viruses and two B viruses. The effectiveness of a vaccine depends on how well it matches the strains that are circulating. If the match is a good one, you can expect the vaccine to be 40 percent to 50 percent effective.

Importance of the flu vaccine

These numbers may seem low but don’t let that fool you. Preventing the flu is only one of the vaccine’s jobs. “It [also] makes it less likely you’ll transmit the virus to others, less likely you will die from the flu and the more people you can vaccinate, the more you can suppress the virus in the community,” notes Dr. Septimus.

Flu vaccines are considered effective if they can reduce hospitalizations and deaths, so a shot can hopefully help reduce the severity of your symptoms if you do get sick. (There’s more than one flu shot; find out which one you should get.)

It’s too early in the season to gauge how well this year’s vaccine and flu strains match, says Dr. Oller. We do know that the Southern Hemisphere seems to be having a relatively mild flu season, probably thanks to people wearing masks and social distancing, she adds.

woman taking temperatureBrothers91/Getty Images

Symptoms of influenza A and B

It doesn’t really matter if you have flu A or B when it comes to how you actually feel if you have the flu. “You feel equally crappy with both of them,” says Dr. Oller. “It used to be the accepted wisdom that flu A was worse and flu B not as bad but that’s recently shown to not be true.”

That means you also can’t know which type of flu you have just based on flu symptoms, which include headache, fever, body aches, and malaise, says Dr. Septimus.

Influenza A and B also have the same chances of complications from the flu, including a potential risk of death. (Beware of these deadly flu signs.)

How are influenza A and B transmitted?

Like SARS-CoV-2, the virus that causes Covid-19, both flu A and B spread from person to person, up to a distance of about six feet. The virus passes through droplets expelled when you sneeze, cough, or even just talk. You can also get the flu by touching something that has the virus on it (like a countertop or phone) then touching your mouth, nose, or eyes, but this isn’t as common.

You can start spreading the virus a day before you have symptoms but you’re most contagious in the first three or four days after symptoms appear.

You can also pass on the virus for up to a week after the flu symptoms—headaches, aches, and other symptoms—have disappeared. “Both A and B are just as contagious and just as easy to spread,” says Dr. Oller.

Treating influenza A and B

When it comes to how to treat the flu, both flu A and B are pretty much the same. “Both are sensitive to Tamiflu,” says Dr. Septimus.

Tamiflu is an antiviral medication that can reduce symptoms and maybe even stop you from getting the flu (at different doses), but it has to be taken early in the course of the illness. You need to take two doses daily for five days, says Dr. Septimus. (Also, here are some natural flu remedies to try.)

A newer drug is baloxavir marboxil (Xofluza). This only needs to be given once, he adds. Beyond these, there’s also rest, pain relievers, and making sure you get plenty of liquids.

The flu vs. coronavirus

The flu virus and coronavirus are two totally different types of virus, and each has many different varieties. Think of it this way. “An apple and an orange are both fruit but completely different fruits,” says Dr. Septimus.

There are other, milder types of coronaviruses besides SARS-CoV-2 that are responsible for cases of the common cold, and they can produce similar symptoms as the flu.

But the Covid-19-causing coronavirus known as SARS-CoV-2 has a wider range of symptoms, says Dr. Septimus. It also takes longer to produce symptoms and is infectious for longer.

Get your flu shot

While Covid-19 vaccine distribution gets up to speed, remember to get your annual flu shot, which is widely available at doctors’ offices, clinics, and retailers nationwide. It’ll protect you against both influenza A and B, which are equally nasty viruses.

Here’s how to get a flu shot near you at nationwide stores and pharmacies:

“If you can reduce the incidence of flu by even 40 percent, that’s [around] 20,000 lives saved,” says Dr. Septimus.” And getting flu shots every year may have a cumulative effect.

“You just keep building up immunity. Most people don’t realize how sick you can get with the flu.” And with Covid-19 in the air, “it’s a deadly combination,” he adds.