More than 500 people at the university's Ann Arbor campus have been diagnosed with the flu—and the majority did not receive the flu vaccine.

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The University of Michigan made headlines this week after school officials announced that the Centers for Disease Control and Prevention (CDC) is investigating a flu outbreak on campus.

The university has seen a "large and sudden increase" in flu cases in students on its Ann Arbor campus, officials said in a news release published November 15. The first positive flu case was detected on campus on October 6, and since then, there have been 528 flu cases diagnosed by the school's University Health Service as of Monday, the university said. Of those, 77% were in people who didn't receive this year's flu vaccine.

Cases have ramped up quickly over the past few weeks: School officials say there were 313 positive cases the week of November 8, and 198 cases detected the week before. Percent positivity rates for testing has been as high as 37% (that means 37% of people who were tested for flu-like symptoms actually had the virus).

CDC Investigating H3N2 Flu Cases on Michigan Campus—What to Know about Influenza A , sick woman sneezing and covering nose with paper napkin, close-up
Credit: Getty Images

Enter, the CDC. University officials said that health experts from the agency are "seeking to learn more about how the flu is spreading and vaccine effectiveness as the nation heads into the flu season" by analyzing the cases on campus.

"We quickly identified these cases as influenza A(H3N2) virus infections," Lindsey Mortenson, University Health Services medical director, said in the November 15 news release. "Partnering with the CDC will accelerate our understanding of how this flu season may unfold regionally and nationally in the setting of the COVID-19 pandemic."

All of this raises plenty of questions about influenza A and why, exactly, it would cause so much spread on the University of Michigan campus in such a short period of time. Here's what you need to know.

What is influenza A—and how is it different than other influenza viruses?

Let's back up a second: There are actually four types of influenza viruses—A, B, C, and D. But human influenza A and B viruses are the ones that cause flu season nearly every winter in the US, the CDC explains. (Influenza C viruses are usually mild and don't cause epidemics, and influenza D viruses mostly infect cattle.)

Influenza A and B viruses are broken into different subtypes. The subtypes for influenza A are broken down based on two proteins on the surface of the virus, the CDC explains: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11).

The virus can go through something called "reassortment," which is when flu viruses swap genetic segments, which can create a lot of different types of influenza A that can circulate. Overall though, the CDC says that the subtypes H1N1 and H3N2 (the one behind the University of Michigan outbreak) are the most common influenza A viruses that circulate in people.

Influenza B, on the other hand, are just broken into two lineages—B/Yamagata and B/Victoria.

"Influenza A and B are just different strains of the virus that have different genetic characteristics," infectious disease expert Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Health.

For the most part, influenza A is the most common type of flu. According to data published in 2016 in the journal PLOS One, influenza A accounts for approximately 75% of flu cases (influenza B makes up the other 25%). And, per CDC data, the 2018–2019 flu season was predominated by influenza A—influenza A(H1N1pdm09) was most common in the beginning of flu season, while influenza a(H3N2) took over for the second part.

When comparing the two strains, "influenza B is generally less severe," Richard Watkins, MD, an infectious disease physician and professor of internal medicine at the Northeast Ohio Medical University, tells Health.

William Shaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine agrees. "Influenza B also usually does not cause big outbreaks," he tells Health. "It usually kind of smolders along and tends to be the most prominent of the viruses at the end of the influenza season."

But that's not a hard and fast rule—and that was seen during the 2019–2020 flu season. CDC data shows that there were two separate waves of flu activity that year, starting off with influenza B viruses. Dr. Shaffner says that, during the 2019–2020 season, influenza B "flummoxed" experts and did the opposite of what it usually does: "It started early and caused outbreaks."

What are the symptoms of influenza A?

Though influenza A and B are two different strains of influenza, they don't really differ in presentation. "Clinically, you cannot tell influenza A and influenza B apart," Dr. Schaffner says. So even if you went to your doctor with flu-like symptoms, they wouldn't be able to evaluate you and tell you which type of flu you have—that requires additional testing through a rapid influenza diagnostic test (RIDT). Those tests may or may not be able to distinguish between influenza A and B—and if they are able to tell which type of influenza virus you have, they cannot determine the subtype, the CDC says.

Essentially, "influenza looks like influenza, no matter which virus caused it," Dr. Schaffner says. Both influenza A and B can cause the following symptoms, per the Mayo Clinic:

  • Fever
  • Aching muscles
  • Chills and sweats
  • Headache
  • Dry, persistent cough
  • Shortness of breath
  • Tiredness and weakness
  • Runny or stuffy nose
  • Sore throat
  • Eye pain
  • Vomiting and diarrhea (more common in children than adults)

How can you prevent or treat influenza A?

You know what we're going to say here: The best thing you can do to prevent influenza A—or influenza B, for that matter—is to get your annual flu vaccine. "The vaccines protect against two different strains of influenza A as well as two different strains of influenza B," Dr. Schaffner says.

Diving deeper, The CDC says this year's flu vaccine protects against the following influenza types and subtypes:

For egg-based vaccines:

  • An A/Victoria/2570/2019 (H1N1) pdm09-like virus;
  • An A/Cambodia/e0826360/2020 (H3N2)-like virus;
  • A B/Washington/02/2019- like virus (B/Victoria lineage);
  • A B/Phuket/3073/2013-like virus (B/Yamagata lineage)

For cell- or recombinant-based vaccines:

  • an A/Wisconsin/588/2019 (H1N1) pdm09-like virus;
  • an A/Cambodia/e0826360/2020 (H3N2)-like virus;
  • a B/Washington/02/2019- like virus (B/Victoria lineage);
  • a B/Phuket/3073/2013-like virus (B/Yamagata lineage).

Other than that, wearing a mask in public and practicing good hand hygiene should dramatically lower your risk of contracting the flu—A and B, Dr. Schaffner says.

If you do happen to get influenza A, however, you should know that treatment is the same as it is for influenza B, Dr. Watkins says. That means your doctor may prescribe flu antiviral drugs like oseltamivir phosphate (aka Tamiflu), zanamivir (Relenza), peramivir (Rapivab), or baloxavir marboxil (Xofluza). Flu antiviral drugs work best when they're started within two days of you getting sick the CDC says, but they may also help beyond that timeframe if you're at a higher risk of serious flu complications.

In general, these medications are known to reduce your fever and flu symptoms, and even shorten the amount of time you're sick by at least a day, the CDC says. Regardless of whether you take antiviral medication or not, the CDC suggests anyone with the flu stay home for at least 24 hours after your fever is gone, without the use of fever-reducing medication.

According to the CDC, most people who get the flu will recover on their own in a few days to two weeks. But if you begin experiencing more severe symptoms from the flu—like difficulty breathing, severe muscle pain or weakness, seizures—you should seek emergency medical treatment.

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