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Pandemic 2009 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 2009 H1N1 influenza is caused by a strain of H1N1 influenza A virus that genetically is a combination of swine, avian, and human influenza viruses.

(See also Influenza.)

Most often, pigs have been infected by strains of influenza that 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.

The novel influenza H1N1 virus, A(H1N1)pmd09, initially referred to as H1N1 swine flu virus, is a combination of swine, avian, and human influenza viruses that spreads easily from person to person. The infection is not acquired through ingestion of pork and is acquired very rarely by contact with infected pigs.

In June 2009, the World Health Organization declared H1N1 swine flu a pandemic. It spread to > 70 countries and to all 50 US states. The majority of the deaths initially occurred in Mexico. The attack rate and mortality for H1N1 swine flu are higher in young and middle-aged adults and lower in older patients than they are for seasonal flu, possibly because younger people lack prior exposure to similar influenza viruses. The pandemic entered the postpandemic period in August 2010. Subsequently, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and distinguish the virus from seasonal H1N1 strains and the 1918 pandemic H1N1 strain. Since 2009, influenza A(H1N1)pdm09 has circulated as a seasonal influenza.

When humans are infected with influenza strains that more closely resemble those that infect pigs, the strains are called swine variant viruses and denoted with a "v" (eg, H3N2v). Over 400 human cases of H3N2v and several cases of H1N1v and H1N2v infections have occurred sporadically in several US states where children and adults have had contact with both sick and apparently healthy domestic pigs at agricultural fairs. There have also been cases of possible human-to-human transmission. The H3N2v virus has genes from avian, swine, and human viruses and the matrix (M) gene from the A(H1N1)pdm09 virus; most cases occurred in the summer of 2012.

Symptoms and Signs of Pandemic 2009 H1N1 Influenza

Symptoms, signs, and complications of pandemic 2009 H1N1 influenza resemble those of ordinary influenza, although nausea, vomiting, and diarrhea may be more common. Symptoms are usually mild, but they can become severe, leading to pneumonia or respiratory failure. Currently circulating isolates appear to have lost some of their initial virulence.

Diagnosis of Pandemic 2009 H1N1 Influenza

  • Sometimes polymerase chain reaction (PCR) testing of respiratory samples

A PCR test can detect the A(H1N1)pdm09 virus in respiratory tract samples (eg, nasopharyngeal swabs, nasal washings, tracheal aspirates). Mildly ill patients do not require testing other than for epidemiologic or surveillance purposes; however, local hospital and public health requirements may vary. Rapid antigen detection tests have decreased sensitivity and generally are clinically useful in diagnosis only if results are positive.

Treatment of Pandemic 2009 H1N1 Influenza

  • Sometimes antiviral drugs

Treatment of pandemic 2009 H1N1 influenza focuses mainly on symptom relief (eg, acetaminophen or ibuprofen for fever and aches).

Antiviral drugs given within 1 to 2 days of symptom onset decrease the duration of fever, severity of symptoms, and time to return to normal activity. Treatment with antiviral drugs is recommended for high-risk patients (including all hospitalized patients) who develop influenza-like symptoms; this recommendation is based on data suggesting that early treatment may prevent complications in these patients.

Drugs for H1N1 influenza are the same as for ordinary influenza strains and include the following:

  • Oseltamivir, zanamivir, and peramivir (neuraminidase inhibitors)

  • Baloxavir (endonuclease inhibitor, new)

Neuraminidase inhibitors interfere with release of influenza virus from infected cells and thus halt spread of infection.

The endonuclease inhibitor baloxavir interferes with viral replication by blocking viral RNA transcription. It is active against influenza A and B and may be an important new treatment option should resistance to neuraminidase inhibitors develop.

Zanamivir is given by an inhaler, 2 puffs (10 mg) 2 times a day; it can be used in adults and children 7 years. Zanamivir sometimes causes bronchospasm and should not be given to patients with reactive airway disease; some people cannot use the inhalation device.

Oseltamivir 75 mg orally 2 times a day is given to patients > 12 years; lower doses may be used in children as young as 1 year. Oseltamivir may cause occasional nausea and vomiting. In children, oseltamivir may decrease the incidence of otitis media; however, no other data clearly show that treatment of influenza prevents complications.

Peramivir is given IV as a single dose and can be used in patients > 2 years who cannot tolerate oral or inhaled drugs. Studies of its use for influenza B are limited.

Baloxavir is given as a single 40 mg dose orally to patients ≥ 12 years and 40 to 80 kg or a single 80 mg dose for patients >80 kg. It can be used in patients ≥ 12 years with uncomplicated influenza who have been symptomatic for ≤ 48 hours and who are otherwise healthy and not at high-risk. It has not been studied in patients who are hospitalized, immunocompromised, or pregnant. or have severe pneumonia.

Adamantanes (amantadine and rimantadine) were previously used; however, more than 99% of current and recent circulating influenza viruses are resistant to adamantanes, so these drugs are currently not recommended for treatment.

Most patients recover fully without taking these drugs.

Prevention of Pandemic 2009 H1N1 Influenza

The current seasonal influenza vaccines are effective against the A(H1N1)pdm09 virus.

Common sense steps (eg, staying home if influenza-like symptoms develop; thorough, frequent handwashing with soap and water or an alcohol-based hand sanitizer) is recommended to reduce the spread of infection.

Key Points about Pandemic 2009 H1N1 Influenza

  • The H1N1 swine flu virus is a combination of swine, avian, and human influenza viruses; it spreads easily from person to person.

  • Testing is not needed if infections are mild unless required by local hospital and public health regulations; PCR testing can detect the A(H1N1)pdm09 virus in respiratory tract samples.

  • Treat symptoms, but use antiviral drugs (eg, oseltamivir, zanamivir, baloxavir) if patients are at high risk of influenza complications or are seriously ill.

  • The current seasonal influenza vaccines are effective against the A(H1N1)pdm09 virus.

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