What You Should Know About H3N2 Flu

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You may have heard the term H3N2 in the news or read about it online. But chances are you don't know much about what it is and how it's different from other types of the flu. Anyone who was around in 2009 is probably familiar with H1N1—the strain of flu that caused a pandemic and sickened millions around the world. But H3N2 is a little different.


H3N2 flu is a subtype of influenza A. Although there are multiple types of influenza, only influenza A is further broken down into subtypes. These subtypes are actually broken down even further as they are identified by the World Health Organization (WHO) based on:

  • The host of origin: The type of animal in which virus was first identified (swine, bird, etc.); for human origin, no host is listed.
  • Geographical origin: Location/city in which the virus was first isolated.
  • Strain number
  • Year of isolation

Each year there are variants of influenza that cause illness during flu season. The virus mutates, making it difficult to predict which one will make people sick each year or even how severe the season will be.

When WHO officials choose the strains of influenza to include in the yearly flu vaccine, they choose two strains of influenza A (one variant of H1N1 and one variant of H3N2) and one or two strains of influenza B. Most flu vaccines contain three strains of influenza, but the quadrivalent vaccine and the nasal spray vaccine, Flu Mist, contain four (two strains of influenza B instead of one).

These strains are chosen over six months before the flu season starts because it takes that long to manufacture and prepare those vaccines for distribution.

H3N2 Flu Epidemics

Although flu symptoms are typically similar no matter the strain of influenza, history has shown that seasons in which H3N2 influenza A is the dominant strain are more severe.

From 2003 to 2013, the three flu seasons that were dominated by H3N2 strains of the flu had the highest mortality rates, causing more deaths on average than other years (excluding the 2009 H1N1 pandemic flu).

At the beginning of the 2014 to 2015 flu season, a mutated version of H3N2 caused a majority of the flu in the United States. The mutated virus was different from the strain of H3N2 influenza A that was included in that season's vaccine.

Unfortunately, that means the vaccine did not provide as much protection against the flu as it would have otherwise. However, that doesn't mean it didn't work at all.


No matter what strain of influenza is circulating each year, you need to know what to expect from the flu. Whether it is caused by H3N2 influenza A or another strain, typical flu symptoms include the following.

common flu symptoms
Illustration by Jessica Olah, Verywell
  • Body aches
  • Fever
  • Headache
  • Sore throat
  • Cough
  • Exhaustion
  • Minor congestion
  • Vomiting and diarrhea (uncommon; occurs more frequently in children)

Diagnosis and Treatment

Only your healthcare provider can diagnose you with the flu. A diagnosis is made based on symptoms you are experiencing, a physical exam, and sometimes a rapid flu test that is performed using a nasal or throat swab.

If your healthcare provider determines that you have the flu, treatment can vary depending on your age, overall health, and length of time that you have been sick.

Antiviral medications, such as Tamiflu or Relenza, can help shorten the severity of your symptoms or the duration of your illness. They are most effective if taken within the first 48 hours of the onset of symptoms.

If you have been sick longer than 48 hours, your healthcare provider may decide that taking them won't really benefit you. You may also be told that you don't need an antiviral medication if you are not at high risk for flu complications.

Even without antiviral medications, there are things you can do to help yourself feel better. Taking over-the-counter medications to alleviate your symptoms, getting plenty of rest, and drinking lots of fluids are important things you can do to give your body a chance to recover. Taking antibiotics won't help unless you have a secondary bacterial infection, as these drugs don't kill viruses.

A Word From Verywell

Influenza is a difficult virus. It changes so frequently that it is hard to identify and treat—and even harder to develop the vaccine months in advance of flu season. H3N2 is one subtype of the influenza virus that often causes significant illness. When it is the dominant strain causing illness during a given year, those flu seasons are often more severe. It's important to take steps to protect yourself and your family from the flu each year, no matter which strain is making people sick. 

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Types of Influenza Viruses.

  2. Centers for Disease Control and Prevention. Selecting Viruses for the Seasonal Influenza Vaccine.

  3. LeMieux J. Some Bad Flu News: H3N2 Is A Major Player This Year. American Council on Science and Health.

  4. Centers for Disease Control and Prevention. Flu Symptoms & Diagnosis.

  5. Centers for Disease Control and Prevention. Flu Treatment.

Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.