What’s the Difference Between Flu A and Flu B?
Flu A and flu B are the most common strains of the flu that circulate in humans.
The U.S. is currently in the middle of flu season, with a high number of cases reported across the country.
There are some key differences between these strains.
The U.S. is officially in the thick of flu season. Data from the Centers for Disease Control and Prevention (CDC) show that a whopping 29% of tests for the flu have come back positive, while nearly 7% of visits to a healthcare provider were for a respiratory illness.
Having the flu is horrible under any circumstances, but there are two major strains to be aware of: Flu A and flu B—and there are some key differences between them.
“They’re both out there at the moment, although the flu A strains are the dominant ones,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine.
Meet the experts: Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York, William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine
A map of flu activity across the country shows that the majority of states have flu activity that’s considered “high” or “very high.” Basically, there’s a lot of flu out there. Here’s what you need to know about flu A vs. flu B.
What’s the difference between flu A and flu B?
Flu A and flu B are two different strains of influenza, explains Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. There are actually four different types of flu—A, B, C, and D—but A and B are the strains that fuel flu season each year, per the CDC.
Just FYI: Flu C infections usually cause mild illness and aren’t linked with epidemics like flu season, while flu D viruses mostly impact cattle.
Flu A circulates in humans and animals, while flu B is primarily considered a human disease, according to Dr. Russo.
Flu A viruses are divided into subtypes based on two proteins on the surface of the virus—hemagglutinin and neuraminidase, according to the CDC. Together, there are 29 different subtypes, creating more than 130 influenza A subtype combinations. However, influenza A subtypes that usually circulate in people are A(H1N1) and A(H3N2).
Flu B viruses aren’t broken into subtypes but are classified into two lineages—B/Yamagata and B/Victoria, per the CDC. Flu B viruses usually change more slowly than flu A viruses and interestingly, Dr. Russo points out that one of the types (B/Yamagata) has not been detected after March 2020.
In general, flu B doesn’t create big epidemics, but flu A does, Dr. Schaffner says. “Flu B kind of smolders along and is usually more prominent at the end of the influenza season,” he says.
What are the symptoms of flu A vs. flu B?
The symptoms can be the same between these two flu strains. However, Dr. Russo says that “flu A usually causes more severe disease than flu B.” Meaning, if you have flu A, your doctor may want to keep a closer eye on you. However, if you’re considered high risk for serious complications of the flu, your doctor will likely want to closely monitor you regardless.
According to the CDC, flu A and flu B symptoms can include:
fever or feeling feverish
cough
sore throat
runny or stuffy nose
muscle or body aches
headaches
fatigue
vomiting and diarrhea (this is more common in children than adults)
How can you know if you have flu A vs. flu B?
Doctors say there’s no way to know if you have flu A or flu B based on symptoms alone. However, a flu test taken at your doctor’s office should be able to tell what strain you have.
Another thing to consider: If there is a lot of flu A or flu B swirling in your area at the moment, and you’re unlucky enough to get the virus, the odds are high that you’ve been infected with the dominant strain. And, again, if you develop a more severe illness, the odds are higher than you have flu A.
Flu A vs. flu B treatment
Treatment for flu A and flu B is the same. “The antivirals that we have—Tamiflu and the like—work well against both A and B,” Dr. Schaffner says.
However, Dr. Russo says that newer antiviral medication baloxavir marboxil (Xofluza) also does well against the flu. Still, these flu treatments aren’t perfect. “Symptoms may improve with these medications a day sooner, at best,” Dr. Russo says.
Supportive care, like getting plenty of rest, making sure to drink lots of fluids, and taking over-the-counter fever-reducing medication like acetaminophen or ibuprofen as needed, may help you feel better while you ride out the flu, Dr. Russo says.
How to prevent flu A and flu B
The best way to prevent both flu A and flu B is to get the influenza vaccine, Dr. Russo says. But how effective the vaccine is at preventing the virus depends on what’s in it.
Each year, the vaccine is tweaked to target certain strains of the virus, including at least one A and one B strain. (This year’s flu vaccine protects against two A strains and one B strain, per the CDC.)
“In any given year, the efficacy may be variable in terms of how perfect the match is,” Dr. Russo says. “Some years, the matches are better for A vs. B, and vice versa.”
If you haven’t gotten your flu shot yet, Dr. Russo recommends scheduling an appointment ASAP. “There’s a lot of flu going on right now and it’s going to be with us for a bit,” he says. “It’s not too late to get vaccinated.”
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