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Pandemic H1N1 Influenza (Swine Flu)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Mar 2021| Content last modified Mar 2021
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Pandemic H1N1(pH1N1) influenza, commonly but inaccurately called swine flu, is a flu infection caused by a relatively new strain of influenza A virus.

(See also Influenza.)

In 2009, there was an epidemic of a new strain of influenza virus that became so widespread that it was considered a pandemic (a major worldwide epidemic). This strain was an H1N1 influenza virus (see types and strains of influenza) that had a combination of genes from pig (swine), bird, and human influenza viruses. Because the first reports focused on the swine component, it was referred to publicly as "swine flu," although it was not acquired directly from pigs. People acquired this influenza virus infection from other infected people (person to person spread) like ordinary flu.

Unlike ordinary flu, pH1N1 flu is more likely to occur in and to cause death in young and middle-aged adults than in older people. This is because the pH1N1 strain was so different from any recent strains of flu.

Pigs can be infected by certain strains of influenza virus, which are more correctly termed "swine flu." Most strains of swine influenza virus are slightly different from those that infect people. These strains very rarely spread to people, and when they do, they very rarely then spread from person to person. However, one variant strain of swine flu virus, H3N2v, has infected children and adults in several U.S. states. Infected people had contact with apparently healthy but infected domestic pigs, usually at agricultural fairs. Also, in a few cases, the virus may have spread from person to person.

Symptoms of Swine Flu

Symptoms of H1N1 influenza are typically flu-like. They include fever, cough, sore throat, body aches, headache, chills, runny nose, and fatigue. Nausea, vomiting, and diarrhea are also common.

In most people, symptoms seem to develop from 1 to 4 days after exposure to the virus and continue for up to another week. People can spread the infection for about 8 days, from the day before symptoms appear until symptoms are gone.

Symptoms are usually mild but can become severe, leading to pneumonia or respiratory failure. The infection can make chronic disorders (such as heart and lung disorders and diabetes) worse and, during pregnancy, can cause complications (such as miscarriage or premature birth).

At high risk of complications are children under 5 years old and people with kidney or liver disorders or a weakened immune system due to drugs or disorders such as AIDS. Severe complications can develop and progress rapidly—sometimes, even in young, healthy people.

Diagnosis of Swine Flu

  • Testing of a sample taken from the nose or throat

If symptoms are mild or typical, particularly when pH1N1 influenza is widespread, testing is usually not required. pH1N1 influenza can be usually be diagnosed based on symptoms and results of a physical examination.

Doctors may take samples of secretions from the nose and mouth for testing. This testing can confirm H1N1 infection.

Prevention of Swine Flu

People with flu-like symptoms should stay home, cover their mouth and nose with a tissue when sneezing or coughing, wash their hands frequently, and use an alcohol-based hand sanitizer.

People who have been in close contact with someone who has pH1N1 flu may be given antiviral drugs.

The current influenza vaccines are effective against pH1N1 influenza virus.

Treatment of Swine Flu

  • Rest and plenty of fluids

  • Relief of symptoms

  • Sometime antiviral drugs

People should see a doctor immediately if they have severe vomiting, shortness of breath, chest or abdominal pain, or sudden dizziness or confusion.

Children should be taken to a doctor immediately if they have blue lips or skin, are not drinking enough fluids, are breathing rapidly or with difficulty, are unusually drowsy or irritable (including not wanting to be held), or have a fever with a rash.

If people at high risk of severe complications (including children under 5 years old) or pregnant women have even mild symptoms, a doctor should be contacted.

Also, if a fever and a worse cough develop after flu-like symptoms disappear in any person, a doctor's attention is required.

Treatment of pH1N1 influenza focuses on relieving symptoms. For example, acetaminophen can relieve fever and aches. Getting enough rest and drinking plenty of fluids can help.

The antiviral drugs oseltamivir, zanamivir, or baloxavir may be used if people are at risk of complications or have severe symptoms. These drugs are most effective when started within 48 hours after symptoms appear. In the United States, most people have recovered from pH1N1 influenza fully without taking these drugs.

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