Introduction

Most people have experienced the flu at some point, with symptoms like fever, chills, coughing, and fatigue. Influenza is caused by a family of viruses classified into four types: A, B, C, and D, with A, B, and C capable of infecting humans.¹ Influenza A is particularly significant due to its ability to cause widespread outbreaks, including those driven by its two major subtypes: H1N1 and H3N2.¹

This article focuses on the H3N2 variant, covering its transmission, symptoms, risk factors, risk management, and prevention.

What is the H3N2 virus, and how does it spread?

H3N2 is a subtype of influenza A, named after two surface proteins: hemagglutinin (H3) and neuraminidase (N2).¹ These viruses frequently mutate to evade the immune system, a process known as reassortment.^1,2 H3N2 emerged when the human H2N2 virus combined with an avian H3N2 strain, triggering the 1968 Hong Kong flu pandemic, which caused an estimated one million deaths.³ Today, it remains a common seasonal flu strain.

Transmission primarily occurs through aerosols released when an infected person coughs, sneezes, or talks, allowing viral droplets to be inhaled by others.⁴ The virus can also survive on surfaces, spreading when individuals touch contaminated objects and then their face. H3N2 has also been linked to pigs, with reports of swine-to-human transmission similar to its H1N1 relative, known as swine flu. People with H3N2 are most contagious within the first 3–5 days of illness.⁴

What symptoms does H3N2 cause?

As an influenza virus, H3N2 presents with typical flu symptoms, including:^5,6

• Fever, chills, and high temperature
• Muscle aches (myalgia)
• Cough and sore throat
• Runny or stuffy nose
• Headache and fatigue
• In some cases, children may also experience vomiting and/or diarrhea

With approximately 1 in 250 H3N2 infections resulting in death, early detection and risk assessment are crucial.

Why managing risks is important

Influenza affects 5–15% of the population annually, leading to 250,000–500,000 deaths worldwide.⁷ H3N2, in particular, has a fatality rate of around 0.4%.⁸ To reduce the risk of infection, especially for vulnerable individuals, proactive precautions and preventive measures are essential.

Who is at risk for H3N2?

Certain individuals are more vulnerable to severe H3N2 infections. If you or someone you know falls into one of the following groups, extra precautions should be taken, especially during flu season:⁹

• Adults over 65 years old
• Pregnant individuals
• Children aged 5 years and under
• People with weakened immune systems (e.g., AIDS, CGD, or other immunodeficiency disorders)
• Individuals with chronic conditions affecting the lungs, heart, or metabolism
• Those undergoing immunosuppressive treatment, such as chemotherapy

Studies show that Black and Native American children are more than twice as likely to require hospitalisation compared to white children, while Black adults face nearly double the risk of hospitalisation compared to white adults.¹⁰

High-risk environments for H3N2 transmission

Certain settings pose a greater risk for H3N2 spread, particularly:^{11, 12}

• Animal/agricultural fairs, farms, and live animal markets
• Hospitals, where influenza outbreaks (termed Hospital-Acquired Influenza) are common
• Workplaces or communities where frequent exposure to infected individuals occurs

Seasonal patterns and environmental factors

H3N2 transmission is influenced by environmental factors such as temperature and humidity, though the exact mechanisms remain unclear.¹³ In the United States, cases peak between November and February.¹⁴ However, in tropical regions, flu patterns vary. India, for instance, experiences two peaks: one from November to February and another during the monsoon season.¹⁵

How to manage H3N2 risks?

Reducing the risk of H3N2 infection is essential, as influenza, though common, can cause significant discomfort and complications and contribute to further spread. Since flu viruses are highly contagious, widespread infections increase the likelihood of larger outbreaks, potentially leading to a pandemic.

Key preventive measures include:^16,17

• Vaccination: Annual flu shots are widely available and strongly recommended for vulnerable populations. However, the vaccine’s efficacy against H3N2 is around 28%
• Hygiene practices: Regular handwashing, use of hand sanitisers, and disinfecting commonly touched surfaces help reduce transmission
• Respiratory etiquette: Covering your nose and mouth when sneezing or coughing (preferably with a tissue or elbow) and wearing a face mask during flu season can lower the risk of spread, particularly for at-risk individuals
• Avoiding crowded spaces: Vulnerable individuals should minimise exposure to crowded places and public transport, especially during peak flu seasons

Implementing these precautions can significantly reduce the risk of infection and limit the spread of H3N2.

Importance of early diagnosis and detection

Early detection of influenza is crucial to minimise the risk of transmission and to facilitate timely treatment, especially for high-risk individuals.

Recognising flu symptoms is the first step in seeking medical attention, depending on an individual's risk for severe illness. Doctors may perform diagnostic tests to confirm influenza and identify the specific strain.¹⁸ These can include:^{7,18, 19}

• Rapid Influenza Diagnostic Tests (RIDTs) are commonly used, but they have relatively low accuracy
• Rapid Molecular Assays, which analyze genetic material, offer more reliable results for diagnosing influenza
• Reverse-transcription polymerase chain reaction (RT-PCR) is often used to identify the specific influenza virus from patient samples
• To detect H3N2 specifically, H3N2 neutralization tests are performed on serum samples taken from the patient

Treatment options for H3N2

For most people, flu symptoms typically last 5-7 days and resolve on their own without complications.⁵ However, at-risk individuals may require more careful management. If preventive measures fail and symptoms develop, antiviral medications can help manage the flu to some extent.

Several antiviral treatments are approved for use against influenza variants, which will be prescribed by your doctor. These include:²⁰

• Oseltamivir (Tamiflu)
• Zanamivir (Relenza)

In both the United Kingdom and the United States, additional antiviral drugs approved for treating variant influenza include:²¹

• Peramivir
• Baloxavir

Antiviral medication is most effective when administered within 1-2 days of symptom onset.

Future outlook

Addressing any viral respiratory illness, including H3N2, requires concerted efforts from both governments and the public to reduce transmission. The challenges of misinformation and vaccine hesitancy were significant during the COVID-19 pandemic and have also impacted flu vaccination efforts.^22,23 Public education and communication efforts are essential to improving vaccine acceptance, with a focus on balancing individual rights and public health needs.

Ultimately, it is everyone’s responsibility to prevent the spread of illness to neighbours, friends, and the broader community.

Future progress hinges on increased research and development of vaccines with higher efficacy. The COVID-19 pandemic underscored the need for improved viral vaccines and sparked global discussions about enhancing vaccine efficacy and coverage. As a result, several initiatives, including mRNA vaccine development, are underway.²⁴ While the fight against H3N2 and influenza continues, ongoing efforts offer hope, and the future of treatments and prevention looks promising.

In the meantime, the preventive tips in this article will help keep you informed and prepared. With time, better treatments and vaccines will become more widely available.

Summary

H3N2 is a strain of influenza that causes the common seasonal flu, characterised by fever, fatigue, cough, runny nose, and headaches. In children, it may also include nausea, vomiting, and diarrhea. People in close contact with birds or pigs, especially during peak seasons, are at higher risk. The virus can lead to severe complications in vulnerable groups, including the elderly, pregnant women, young children, and those with chronic or immunocompromising conditions.

To reduce risks, vulnerable individuals should get vaccinated, avoid crowded places during flu season, and maintain good hygiene. Early diagnosis and treatment with antiviral medications can help manage symptoms. With proper precautions, the flu usually resolves within a week with minimal long-term effects.