What You Should Know About H3N2 Flu

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 flu. Adults who were around in 2009 are probably familiar with H1N1—the strain of flu that caused a pandemic and sickened millions around the world. But H3N2 is a little different.

This article discusses what H3N2 flu is, as well as its symptoms and how it's diagnosed and treated.

Flu Overview

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 the 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 the 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.

What Is the H3N2 Flu Virus?

H3N2 virus is a non-human influenza virus that normally circulates in pigs but can infect people. Viruses that normally circulate in pigs are “swine influenza viruses.” When these viruses infect humans, they are termed “variant” viruses.

The H3N2 virus was circulating in pigs in 2010 and was first detected in people in 2011. Since first being identified in humans in 2011, the H3N2 virus has caused infection in people yearly throughout the U.S.

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.

H3N2 is not as severe as H1N1 but may be more severe than some other strains of flu. 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 in 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.

H3N2 Flu Symptoms

No matter what strain of influenza is circulating each year, you need to know what to expect from the flu. H3N2 virus infection usually lasts between five and seven days but can cause a severe cough that lasts up to three weeks. Whether it is caused by H3N2 influenza A or another strain, typical flu symptoms include:

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)

Risk Factors for Complications

People who are at higher risk for developing flu-related complications include:

  • Children younger than 5 years
  • People 65 years and older
  • People who are pregnant
  • People with certain long-term health conditions (like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions)

H3N2 Flu 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 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 antiviral medications won't really benefit you. You may also be told you don't need an antiviral medication if you are not at high risk for flu complications.

Even without antiviral medications, there are important things you can do to give your body a chance to recover, such as:

  • Taking over-the-counter (OTC) medications to alleviate your symptoms
  • Getting plenty of rest
  • Drinking lots of fluids

Taking antibiotics won't help unless you have a secondary bacterial infection, as these drugs don't kill viruses.

Ways to Prevent H3N2 Flu

Steps you can take to reduce the infection and spread of H3N2 between people are the same as other flu viruses:

  • Cover your nose and mouth with a tissue or inside of your elbow when you cough or sneeze. Throw the tissue in the trash after you use it.
  • Wash your hands often with soap and water, especially immediately after you cough or sneeze. If soap and water are not available, an alcohol-based hand rub may be used.
  • Avoid touching your eyes, nose, or mouth. Germs spread that way.
  • Try to avoid close contact with sick people.
  • If you are sick, stay home from work or school until your illness is over.

These steps can help prevent the spread of flu between pigs and people:

  • Avoid pigs and swine barns at the fair if you are at high risk of serious flu complications
  • Don’t take food or drink into pig areas; don’t eat, drink or put anything in your mouth in pig areas
  • Don’t take toys, pacifiers, cups, baby bottles, strollers, or similar items into pig areas
  • Wash your hands often with soap and running water before and after exposure to pigs. If soap and water are not available, use an alcohol-based hand sanitizer.
  • Avoid close contact with pigs that look or act ill.
  • Wear personal protective equipment—like protective clothing, gloves, and masks that cover your mouth and nose—when you must come in contact with pigs that are known or suspected to be sick.
  • Minimize contact with pigs in the pig barn and arenas

Summary

H3N2 is one subtype of the influenza A virus that often causes significant illness. Flu seasons are often more severe when H3N2 is the dominant strain causing illness. Because the influenza virus changes so frequently it is hard to identify and treat—and even harder to develop a vaccine months in advance of flu season.

Symptoms are similar no matter what strain of influenza is circulating each year. Treatment may include antiviral medications if diagnosed early; otherwise, rest, lots of fluids, and OTC medications can help relieve symptoms.

9 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. Centers for Disease Control and Prevention. Influenza A (H3N2) variant virus.

  4. Oklahoma State Department of Health. Influenza A H3N2 variant (H3N2v).

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

  6. Jester BJ, Uyeki TM, Jernigan DB. Fifty Years of Influenza A(H3N2) Following the Pandemic of 1968Am J Public Health. 2020;110(5):669-676. doi:10.2105/AJPH.2019.305557

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

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

  9. Centers for Disease Control and Prevention. Healthy habits to help protect against flu.

Additional Reading

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