H1N1 (Swine Flu): Causes, Symptoms, Treatment, and Prevention 

Introduction

H1N1, also known as swine flu, is a type of influenza A virus that was identified in humans in 2009. It emerged as a result of a mix of genes from flu viruses found in North American swine, North American birds, humans, and Eurasian swine¹.   

H1N1 causes respiratory illness by infecting the nose, throat, and sometimes the lungs. Its symptoms are similar to those of seasonal flu, which includes, fever, cough, sore throat, and body aches. In India, signidicant H1N1 outbreaks were recorded in 2009 and 2015, and the virus continues to circulate as a seasonal flu strain².   

The H1N1 infection can affect both the upper and lower respiratory tract. In severe cases, it may lead to complications such as viral pneumonia and acute respiratory distress syndrome (ARDS), particularly among vulnerable groups such as pregnant women, young children, the elderly, and individuals with compromised immune systems. 

Vaccines that offer protection against the H1N1 virus are widely available and are included in the annual influenza vaccine recommended by global and national health authorities. 

Understanding H1N1 causes, symptoms, treatment, and preventive measures is critical for managing future outbreaks and minimising health impacts. This article provides a comprehensive overview of H1N1 influenza to help increase awareness. 

What Is Swine Flu (H1N1)?

Swine flu, or H1N1 influenza, is a contagious viral infection caused by a subtype of the influenza A virus, primarily affecting the respiratory tract. The H1N1 virus originated from a mix of swine, bird, and human flu viruses and gained global attention during the 2009 pandemic³. 

It is important to note that, although it is called “swine flu,” the virus primarily spreads through inhaling contaminated air (either from an infected person or animal) and not through the consumption of properly cooked pork^3,4.  

Causes of Swine Flu (H1N1)

Swine flu (H1N1) is caused by the influenza A (H1N1) virus³.  

Infection occurs when the virus enters the body through the nose, mouth, or eyes. The most common sources of infection include: 

• Inhalation of respiratory droplets released when an infected person coughs, sneezes, or talks³. 
• Touching contaminated surfaces or objects and then touching the face (especially nose, mouth, or eyes)³. 

While the original H1N1 strain emerged from swine and was occasionally linked to close contact with infected pigs, the 2009 pandemic strain has since adapted to human transmission. In most incidences, it is not spread from pigs to humans. Instead, the virus primarily spreads through droplet transmission and fomite transmission, making person-to-person contact and surface hygiene key areas of prevention. 

Symptoms of Swine Flu (H1N1)

Swine influenza (H1N1) presents symptoms similar to seasonal influenza, ranging from mild to severe. Common H1N1 symptoms include³:

• Fever or chills  
• Sweating 
• Cough 
• Sore throat 
• Runny or stuffy nose 
• Appetite loss 
• Weight loss 
• Body aches or muscle pain 
• Headache 
• Fatigue or weakness 
• Shortness of breath (in some cases) 

Gastrointestinal H1N1 symptoms such as nausea, vomiting, or diarrhoea are more commonly observed in children⁶. 

Risk Factors

While swine influenza (H1N1) can affect anyone, certain individuals are at a higher risk. These risk factors include⁷: 

• Children under 5 years of age 
• Adults over 65 years of age 
• Pregnant women 
• People with chronic health conditions, such as: 
• Asthma 
• Diabetes 
• Heart disease 
• Chronic lung diseases (such as COPD and interstitial lung disease) 
• Individuals with weakened immune systems (e.g., due to cancer, HIV/AIDS, or immunosuppressive therapy) 
• Obese individuals 
• Residents of nursing homes or long-term care facilities 

How Is Swine Flu (H1N1) Diagnosed?

Swine flu (H1N1) is diagnosed by detecting the presence of the H1N1 virus in respiratory specimens that are typically collected using nasopharyngeal swabs, nasal swabs, or nasal aspirates, depending on the type of diagnostic test being used. Throat swabs, while sometimes collected, are generally less sensitive and not preferred for molecular testing methods such as RT-PCR, which is commonly used for confirming H1N1 infection. Early and accurate diagnosis helps guide H1N1 treatment and prevent complications.  

Common diagnostic methods include⁸: 

• Rapid influenza diagnostic tests (RIDTs): Provide quick results by detecting viral antigens, but have limited sensitivity, often ranging from 50–70%, especially in adults and outside peak flu season. RIDTs usually cannot distinguish between influenza types or subtypes (e.g., H1N1 vs. H3N2).  
• Reverse transcription polymerase chain reaction (RT-PCR): Highly accurate and considered the gold standard. It detects the virus’s genetic material (RNA) and can specifically identify the H1N1 strain. 
• Viral culture: Confirms the presence of the virus by growing it in a laboratory. While reliable, it is time-consuming and mainly used for research or confirmation purposes. 

Complications of Swine Flu (H1N1)

While many cases of swine flu (H1N1) are mild, the infection can lead to serious complications, especially in high-risk individuals. These complications may require hospitalisation and can sometimes be life-threatening. Common complications include^3,9: 

• Pneumonia (viral or secondary bacterial) 
• Acute Respiratory Distress Syndrome  
• Otitis media (ear infection)  
• Sinusitis 
• Worsening of chronic medical conditions such as asthma, diabetes, or heart disease 
• Neurological complications, including: 
• Seizures (rare) 
• Encephalopathy 
• Encephalitis 
• Guillain-Barré syndrome 
• Transverse myelitis 
• Sepsis (a life-threatening response to infection) 
• Multi-organ failure (in severe cases) 

Treatment and Management

The H1N1 treatment focuses on reducing the severity of H1N1 symptoms, preventing complications, and stopping the spread of the H1N1 virus. Management strategies include: 

• Antiviral medications: Doctors commonly prescribe oseltamivir, zanamivir, and peramivir for symptom control. They are most effective when started within 48 hours of symptom onset³.  

Zanamivir is inhaled and not recommended for patients with asthma or COPD due to the risk of bronchospasm. Peramivir is administered intravenously and is usually reserved for hospitalised patients or those unable to take oral medications. 

• Supportive care: Adequate rest and hydration are necessary when recovering from swine flu. You may even use fever-reducing medications and other symptom-relieving H1N1 treatments, as needed under medical guidance³. 
• Monitoring for complications: Watch for signs of worsening symptoms such as difficulty breathing, persistent chest pain, or high fever.  

Prevention

Practicing essential H1N1 precautions, such as those mentioned below, can significantly reduce the risk of infection: 

• Annual flu vaccination: The seasonal flu vaccine includes protection against H1N1 and is recommended for everyone over 6 months of age³. 
• Frequent handwashing: Wash hands thoroughly with soap and water (for a minimum of 20 seconds) or use an alcohol-based hand sanitiser¹⁰. 
• Covering coughs and sneezes: Use a tissue (or elbow) to cover your mouth and nose when coughing or sneezing¹⁰. 
• Avoiding close contact with sick individuals/animals: Stay away from people or pigs showing flu-like symptoms^5,10. 
• Staying home when sick: Helps prevent the spread of the virus to others¹⁰. 
• Disinfecting frequently touched surfaces: Clean surfaces like doorknobs, phones, and countertops regularly¹¹. 
• Using a face mask (when necessary): Especially in crowded places or when taking care of someone with flu symptoms¹¹.
• Maintaining healthy habits: Support your immune system by getting enough sleep, staying physically active, managing stress, drinking plenty of fluids, and eating nutritious foods¹¹. 

Living with Swine Flu (H1N1)

If you are diagnosed with swine influenza (H1N1), it’s important to manage your symptoms at home while taking steps to prevent spreading the virus to others. Key guidelines for daily living while ill include: 

• Stay home and self-isolate when ill, preferably in a separate room, until at least 24 hours after the fever has resolved without the use of fever-reducing medications; immunocompromised individuals may require a longer isolation period as per health authority recommendations. 
• Monitor your symptoms daily and seek medical help if you experience worsening symptoms. 
• Get plenty of rest and drink fluids to stay hydrated and support recovery³. 
• Cover your mouth and nose when coughing or sneezing¹⁰. 
• Dispose of used tissues properly and wash hands frequently¹⁰.
• Wear a mask when around people to reduce the risk of transmission¹¹.
• Return to work/school only after you have fully recovered for at least 24 hours without medication¹¹. 

The Role of Vaccination in H1N1 Prevention

Vaccination is one of the most effective ways to prevent H1N1. 

Seasonal influenza vaccines, including quadrivalent vaccines (which also protect against both lineages of influenza B), are designed to provide immunity against the most common flu strains, including H1N1 and H3N2 (both subtypes of influenza A)¹². 

• The vaccine is recommended annually for everyone aged 6 months and older³. 
• It is especially important for high-risk groups such as young children, the elderly, pregnant women, and individuals with chronic medical conditions³.
• Vaccination helps reduce the risk of H1N1 infection, severe illness, hospitalisation, and complications; quadrivalent influenza vaccines protect against H1N1, H3N2 (influenza A subtypes), and two influenza B lineages, Victoria and Yamagata. 

When to See a Doctor

Most cases of swine flu (H1N1) are mild and can be managed at home. However, medical attention should be sought if you experience the following¹⁰: 

• Difficulty breathing or shortness of breath 
• Persistent chest pain or pressure 
• High fever (≥102°F or 39°C) lasting more than 3 days despite the use of antipyretics 
• Severe or worsening cough 
• Confusion or dizziness 
• Bluish lips or face 
• Symptoms that improve but return with worsening fever or cough 

Note: In children, watch for signs like fast breathing, irritability, poor feeding, or lack of responsiveness¹⁰. Additionally, pregnant women, the elderly, and people with underlying health conditions should consult a doctor early, even for mild symptoms.  

Conclusion

Swine flu (H1N1) remains a significant public health concern, particularly for vulnerable populations. The H1N1 (pdm09) strain, which caused the 2009 pandemic, is now part of regular seasonal influenza circulation and no longer classified as a separate pandemic strain. Awareness of symptoms, early detection, and timely medical treatment are essential to reducing the severity and spread of the infection. Annual vaccination remains the most effective preventive measure.  

In addition, following basic H1N1 precautions, such as maintaining hand hygiene, using masks, and avoiding close contact with those who are ill can reduce transmission. Public health campaigns also play a vital role in reinforcing these preventive measures, especially during flu season and among high-risk groups. 

Frequently Asked Questions (FAQs)

References

1. World Health Organization. Influenza A (H1N1) [Internet]. World Health Organization; [cited 2025 Jun 19]. Available from: https://www.who.int/emergencies/situations/influenza-a-(h1n1)-outbreak 

2. Kshatriya RM, Khara NV, Ganjiwale J, Lote SD, Patel SN, Paliwal RP. Lessons learnt from the Indian H1N1 (swine flu) epidemic: Predictors of outcome based on epidemiological and clinical profile. J Family Med Prim Care. 2018 Nov-Dec;7(6):1506-1509. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6293944/ 

3. Jilani TN, Jamil RT, Nguyen AD, Siddiqui AH. H1N1 Influenza [Internet]. StatPearls Publishing; [updated 2024 Mar 4; cited 2025 Jun 19]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513241/ 

4. Centers for Disease Control and Prevention (CDC). About Influenza in Swine [Internet]. CDC; [cited 2025 Jun 19]. Available from: https://www.cdc.gov/swine-flu/about/influenza-in-swine.html 

5. Centers for Disease Control and Prevention (CDC). What People Who Raise Pigs Need To Know About Influenza (Flu) [Internet]. CDC; [cited 2025 Jun 19]. Available from: https://www.cdc.gov/swine‑flu/media/pork‑producer‑fact‑sheet.pdf 

6. Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016 Dec 7;355:i6258. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5141587/ 

7.  Coleman BL, Fadel SA, Fitzpatrick T, Thomas SM. Risk factors for serious outcomes associated with influenza illness in high- versus low- and middle-income countries: Systematic literature review and meta-analysis. Influenza Other Respir Viruses. 2018 Jan;12(1):22-29. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5818335/ 

8. R R, Dalal A, Mohan H, Prasad M, Pundir CS. Detection methods for influenza A H1N1 virus with special reference to biosensors: a review. Biosci Rep. 2020 Feb 28;40(2):BSR20193852. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC7000365/ 

9. Gutierrez C, Nazar GA, Torres JP. Otolaryngological complications in patients infected with the influenza A (H1N1) virus. Otolaryngol Head Neck Surg. 2012 Mar;146(3):478-82. Available from: https://pubmed.ncbi.nlm.nih.gov/21998086/ 

10. Department of Health & Family Welfare, Government of Puducherry. Swine Influenza (A H1N1) [Internet]. Puducherry: Government of Puducherry; [cited 2025 Jun 19]. Available from: https://health.py.gov.in/swine-influenzaa-h1n1 

11. Centers for Disease Control and Prevention. Healthy Habits to Prevent Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/prevention/actions-prevent-flu.html 

12. Trombetta CM, Kistner O, Montomoli E, Viviani S, Marchi S. Influenza Viruses and Vaccines: The Role of Vaccine Effectiveness Studies for Evaluation of the Benefits of Influenza Vaccines. Vaccines (Basel). 2022 May 1;10(5):714. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9143275/ 

13. Centre for Food Safety, Hong Kong. When buying and handling pork: food safety advice during swine flu [Internet]. Hong Kong: CFS; [cited 2025 Jun 19]. Available from: https://www.cfs.gov.hk/english/faq/faq_16_Leaflet_on_swine_flu_html.html 

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