What are the different types of influenza?

Types of Influenza Viruses

Influenza viruses are categorized into three main types: influenza A, influenza B, and influenza C, with influenza A and B being responsible for epidemic human disease and causing the most significant morbidity and mortality worldwide. 1, 2

Influenza A

• Influenza A viruses are further categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N) 1
• Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have been in global circulation 1
• There are currently 15 distinct hemagglutinin (H1-H15) subtypes and 9 neuraminidase (N1-N9) subtypes identified, which can exist in various combinations 3, 4
• Influenza A viruses can infect a wide variety of species including humans, pigs, horses, sea mammals, and birds 4
• Influenza A viruses undergo more rapid antigenic changes (antigenic drift) than influenza B viruses, leading to frequent seasonal epidemics 1
• More dramatic changes called "antigenic shift" can occur in influenza A, resulting in pandemic potential when a new subtype emerges 1, 4
• Influenza A (H3N2) infections tend to cause more severe symptoms with higher fever, more pronounced leukopenia, and elevated C-reactive protein compared to A (H1N1) or influenza B 5

Influenza B

• Influenza B viruses are not categorized into subtypes but are separated into two distinct genetic lineages: Yamagata and Victoria 1
• Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses 1
• Influenza B typically affects younger populations compared to influenza A (H3N2) 5
• Gastrointestinal symptoms such as nausea, epigastric pain, and diarrhea are more prominent in influenza B infections 5
• Influenza B viruses only infect humans and seals, unlike influenza A which has a broader host range 4, 6

Influenza C

• Influenza C viruses cause mild respiratory illness and are not typically associated with epidemics 4, 2
• There is no provision for describing distinct subtypes of influenza C viruses in the nomenclature system 3

Clinical Presentation Differences

• Influenza A (H3N2) typically presents with higher fever (averaging 38.6°C) compared to A (H1N1) or B 5
• Myalgia and symptoms such as headache, general malaise, and sore throat are common across all influenza types 5
• Uncomplicated influenza illness is characterized by abrupt onset of constitutional and respiratory symptoms including fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis 1
• The incubation period for influenza is 1-4 days, with an average of 2 days 1
• Persons can be infectious starting the day before symptoms begin through approximately 5 days after illness onset; children can be infectious for a longer period 1

Epidemiological Significance

• Influenza A viruses are primarily responsible for pandemics due to their ability to undergo antigenic shift 4, 2
• Aquatic birds are considered the evolutionary hosts of influenza viruses and harbor many distinct subtypes of influenza A 4, 6
• Annual epidemics of influenza typically occur during fall or winter months, but peak activity can occur as late as April or May 1
• Currently, epidemics occur worldwide due to infection with influenza A viruses of subtypes H1N1 and H3N2 or with influenza B virus 4

Diagnostic Considerations

• For most patients in the outpatient setting, the diagnosis of influenza is made clinically, and laboratory confirmation is not necessary 2
• Rapid molecular assays are the preferred diagnostic tests because they can be done at the point of care, are highly accurate, and provide fast results 2
• Laboratory testing may be useful in hospitalized patients with suspected influenza and in patients for whom a confirmed diagnosis will change treatment decisions 2

Treatment Implications

• Antiviral medications can be considered if the patient presents within 48 hours of symptom onset, with greatest benefit when started within 24 hours 2
• Most seasonal influenza A (H1N1) virus strains tested from the United States and other countries have developed resistance to oseltamivir 1
• Annual influenza vaccination remains the cornerstone for prevention of influenza and its complications 1, 2