The specific type typically doesn't show up until springtime.

By Leah Groth
January 13, 2020

The US is only about three months into the 2019–2020 flu season, and the illness has already left its mark—but it's a little different this year.

According to the Centers for Disease Control and Prevention, since October 2019, 9.7 million flu illnesses, 87,000 hospitalizations, and 4,800 deaths from the flu have been reported. These numbers are relatively consistent with past years, but there is one major difference between this flu season and most in recent history: There has been an increase in the number of cases involving influenza B—so much so that it is dominating flu season for the first time in nearly 30 years. 

Usually, the majority of flu cases—a whopping 75 percent of reported cases—are influenza A. However, this season, influenza B is spreading like wildfire—the CDC even reported that this is the first time the US has seen it predominate flu season since the 1992–1993 flu season, with 21 of the 32 pediatric deaths during the current season being due to influenza B, per the CDC

RELATED: 5 Flu Symptoms to Watch Out for—And How to Make Them Go Away ASAP

What exactly is influenza B—and how is it different than influenza A?

It's important to remember that there are actually four types of influenza viruses: A, B, C, and D. Flu viruses A and B, however, are the only ones thought to cause the seasonal flu epidemics each winter (influenza C is typically mild, and influenza D affects cattle), according to the CDC.

Going a little deeper, influenza A and B viruses are each divided into different subtypes. Influenza A viruses are the trickiest: They're divided into subtypes based on two specific proteins, hemagglutinin (H) and neuraminidase (N)—ultimately, influenza A viruses can broken down into 198 different subtype combinations, per the CDC. Influenza B viruses, however, are classified into two lineages: B/Yamagata and B/Victoria.

A big difference between influenza B and influenza A is that influenza B typically comes after influenza A, hitting later in the season around springtime.

But as far as symptoms go, influenza A and B are quite alike. "There are more similarities than there are differences between influenza A and influenza B," Richard Martinello, MD, a Yale Medicine infectious disease expert, explains to Health. While in the laboratory they can be distinguished, from a clinical perspective, they can cause essentially the same typical influenza we usually think about. 

According to the CDC, the symptoms for influenza B are similar to other strains, including fatigue, cough, fever (or chills), a sore throat, runny nose, muscle aches, headache, and gastrointestinal problems.

Another key difference between influenza A and B? While influenza B viruses are typically less common than influenza A viruses, influenza B infections can be more severe in children, and can lead to complications that require hospitalization or death.

RELATED: How Long Does the Flu Last?

So how can you prevent influenza B?

According to the CDC, the best way to prevent the flu—both influenza A and B—is by getting the flu vaccine. And no, it isn’t too late in the season.

"CDC recommends that everyone 6 months of age and older get a seasonal flu vaccine each year by the end of October,” they write. “However, as long as flu viruses are circulating, vaccination should continue throughout flu season, even in January or later."

Additionally, Dr. Martinello suggests doing your best to avoid people who may be sick with the flu, and washing your hands frequently. Also, if you get sick or your children are infected, make sure to avoid exposing other people to the illness.

To get our top stories delivered to your inbox, sign up for the Healthy Living newsletter

Advertisement