What is the H1N1 flu?

H1N1 Influenza: Characteristics and Significance

H1N1 is a subtype of influenza A virus that has caused both seasonal influenza and the 2009 pandemic (swine flu). 1, 2

Influenza A H1N1 Classification and Origin

• H1N1 is a subtype of influenza A virus, categorized based on two surface antigens: hemagglutinin (H) and neuraminidase (N) 1
• The 2009 pandemic H1N1 strain (swine flu) was a novel influenza A virus that emerged in April 2009 in Mexico and the United States 1, 3
• The 2009 H1N1 pandemic virus contained a mixture of genetic material from human, swine, and avian (bird) influenza viruses, making it antigenically distinct from human influenza A (H1N1) viruses that had circulated since 1977 1, 4
• The HA gene of the 2009 H1N1 pandemic virus is most closely related to influenza A viruses circulating among pigs during preceding decades and is believed to have evolved from the avian-origin 1918 pandemic influenza A(H1N1) virus 1

Epidemiology and Transmission

• H1N1 influenza is transmitted primarily through respiratory droplets or contact with contaminated fomites 3, 5
• The incubation period for H1N1 influenza is typically 2-7 days 3
• During the 2009 H1N1 pandemic, the virus disproportionately affected younger populations compared to seasonal influenza 1
• The median age of persons with laboratory-confirmed 2009 H1N1 infections was 12 years, with highest infection rates among persons aged 5-24 years 1
• Unlike seasonal influenza, which typically causes highest mortality in those over 65, the 2009 H1N1 pandemic had a median age of 37 years among fatal cases 1

Clinical Presentation

• H1N1 influenza typically presents with symptoms similar to other influenza strains: fever, cough, sore throat, and myalgia 1, 3
• A distinguishing feature more commonly seen with 2009 H1N1 was gastrointestinal symptoms including diarrhea and vomiting 3, 6
• Most cases of H1N1 influenza are mild and self-limiting, with less than 10% requiring hospitalization 3
• High-risk groups for severe disease include children younger than 5 years, pregnant women, individuals with chronic medical conditions, and adolescents on aspirin therapy 1, 7

Diagnosis

• Definitive diagnosis of H1N1 requires specific testing using real-time reverse transcriptase-polymerase chain reaction (RT-PCR) or viral culture 1, 3
• Rapid influenza diagnostic tests have limited sensitivity (40-70%) for detecting H1N1 and negative results should not be used to exclude diagnosis 1

Treatment and Prevention

• The primary prevention strategy for H1N1 influenza is annual vaccination 1, 2
• During the 2009 pandemic, a specific monovalent H1N1 vaccine was developed, but the strain has since been incorporated into seasonal influenza vaccines 1
• For the 2010-2011 influenza season and beyond, the A/California/7/2009 (H1N1) strain (derived from the 2009 pandemic virus) replaced the previous seasonal H1N1 strain in the trivalent vaccine 1
• Antiviral medications effective against H1N1 include neuraminidase inhibitors such as oseltamivir and zanamivir, though resistance patterns may vary 3, 4

Clinical Significance

• The 2009 H1N1 pandemic resulted in significantly higher pediatric mortality than typical seasonal influenza, with nearly 4 times the average number of laboratory-confirmed influenza-associated pediatric deaths reported during the 2009-2010 season compared to the previous 5 seasons 1
• While most cases of H1N1 are mild, severe manifestations can include pneumonia and respiratory failure 3
• In pregnant women, H1N1 infection has been associated with increased risk of complications, particularly in the second and third trimesters 7

H1N1 influenza continues to circulate as a seasonal influenza strain, and ongoing surveillance, vaccination, and appropriate antiviral use remain important components of influenza management and prevention 2, 4.