What Is Influenza A Virus Subtype H3N2 and How Is It Detected?
Although vaccines have long been available to mitigate its impact, influenza continues to cause a significant global burden on human health and healthcare capacity. Alongside annual epidemics, influenza A viruses have caused several pandemics since the turn of the 20th century. Throughout the last decade, H3N2 has tended to dominate in prevalence and severity both over different subtypes of influenza A, such as H1N1 and H1N2, as well as influenza B. However, with the growing H3N2 outbreak in India that has recently led to two deaths, national and global public health agencies are warning the world that the flu appears to have returned to its pre-pandemic activity following two historically quiet seasons. Read along to learn more about influenza A Virus subtype H3N2, its symptoms, and detection.
What Is Influenza A Virus Subtype H3N2?
Influenza viruses are RNA viruses of the family Orthomyxoviridae. They are classified into four main types depending on their different antigenic and biological properties. Among these, influenza A and influenza B are the only types to cause human disease of any concern. Influenza C and influenza D, on the other hand, have been associated with mild to no disease in human populations. While the impact of influenza B has been limited to epidemics with mild to moderate disease, influenza A has accounted for all influenza pandemics and the majority of the influenza epidemics in human history. In addition, many of the deadliest influenza outbreaks, such as the Spanish Flu (1918), Asian Flu (1957), Hong Kong Flu (1968), Bird Flu (2004), and Swine Flu (2009), have been linked to different subtypes of influenza A.
H3N2 is one of the many subtypes of Influenza A. Similar to other subtypes of influenza A, it is named according to the forms of the two major proteins located on the surface of its coat, hemagglutinin (H) and neuraminidase (N). Compared to other subtypes of influenza A and influenza B, this subtype has been associated with more severe disease, higher rates of hospitalization, and increased mortality among all age groups. In fact, with the exception of the 2009 H1N1 pandemic flu, the three influenza seasons dominated by H3N2 had the highest mortality rates among all influenza seasons between 2003 and 2013. Between 1968 and 1969, influenza A subtype H3N2 caused the Hong Kong Flu, a pandemic estimated to kill more than 1 million people worldwide. In 2003, the subtype caused the Fujian human flu, leading to significantly more hospitalizations and death than in a typical flu season.
How Common Is Influenza A Virus Subtype H3N2?
Seasonal influenza outbreaks commonly occur as regular annual epidemics in the temperate regions of both the northern and the southern hemispheres. In the equatorial areas, influenza outbreaks can be experienced all year round. Although relatively rare and typically imported from tropical regions, sporadic influenza cases also occur. Altogether, influenza is a common and widespread infection that has affected human populations for hundreds of years and continues to affect millions of people yearly. Among the different strains of influenza that cause human disease, influenza A subtype H3N2 has remained largely dominant in prevalence over H1N1, H1N2, and influenza B in the last decade. Alongside its prevalence, the strain has also caused relatively higher hospitalization and mortality rates during the influenza seasons that it had dominated.
What Are the Symptoms of Influenza A Virus Subtype H3N2?
Although H3N2 has been associated with higher hospitalization and mortality rates, infections with this subtype do not present with any unique symptoms. Regardless of the responsible subtype, it is estimated that up to 75% of all cases of seasonal influenza during a typical season are asymptomatic. Symptomatic influenza cases usually present with fever, runny or congested nose, sore throat, cough, headache, myalgia, and headache that develop with a rapid onset. Gastrointestinal symptoms such as stomachache, nausea, vomiting, and diarrhea are also common in children infected with influenza. Most people with influenza recover in a few days without medical attention. However, the infection can lead to severe disease and complications in people with low or compromised immunities, such as infants, older adults, pregnant women, people with underlying medical conditions, and those on treatment or therapy for certain diseases.
How Is Influenza A Virus Subtype H3N2 Detected?
As the seasonal pattern, disease onset, and clinical manifestations of influenza can mimic that of various other respiratory infections, such as the common cold, RSV, and COVID-19, symptoms alone may not be sufficient to inform a diagnosis. Nevertheless, diagnostic testing for influenza may be highly beneficial, especially when the results can shape important clinical decisions, such as whether to start antiviral treatment or to implement infection prevention and control measures. Therefore, according to the Centers for Disease Control and Prevention (CDC), influenza testing is also advised for all patients with suspected influenza admitted to the hospital.
Diagnostic tests available for the detection of influenza viruses range from molecular assays such as reverse transcription polymerase chain reaction (RT-PCR) testing, rapid molecular testing, and other forms of nucleic acid amplification tests (NAATs) to antigen-detection assays such as rapid immunofluorescence tests. Although viral culture is also a possible detection method, it is not commonly used as it needs to provide timely results to inform clinical management. Antigen-detection assays, such as our Influenza A/B Rapid Test Kit, RapidFor™ SARS-CoV-2 & FLU A/B Antigen Combo Test Kit, and SARS-CoV 2 + FLU A/B + RSV Combo Test Kit allow for the detection and differentiation of influenza A/B infections in human respiratory specimens. Coming in a lateral flow assay format, these kits utilize multiple sets of antibodies to target and capture pathogen-specific antigens. Regardless of the responsible subtype, these assays deliver highly sensitive and accurate results in just 15 to 20 minutes.
Chartrand, C., Minion, J., & Pai, M. (2010). Rapid diagnostics for influenza: what are the options? Future Microbiology. https://doi.org/10.2217/fmb.10.120
Ghebrehewet, S., MacPherson, P., & Ho, A. (2016). Influenza. BMJ, i6258. https://doi.org/10.1136/bmj.i6258
Harrington, W. N., Kackos, C., & Webby, R. J. (2021). The evolution and future of influenza pandemic preparedness. Experimental and Molecular Medicine, 53(5), 737–749. https://doi.org/10.1038/s12276-021-00603-0
Krammer, F., Smith, G. J. D., Fouchier, R. a. M., Peiris, J. S. M., Kedzierska, K., Doherty, P. C., Palese, P., Shaw, M. L., Treanor, J. J., Webster, R. G., & García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers, 4(1). https://doi.org/10.1038/s41572-018-0002-y