Influenza A: What is It, Causes, Symptoms, Treatment & Prevention

Introduction

Each year, influenza affects an estimated one billion people worldwide, with 3 to 5 million cases resulting in severe illness and between 290,000 and 650,000 deaths due to respiratory complications (World Health Organization, 2023)¹.  

Influenza is primarily classified into four types (A, B, C, and D). Among them, some influenza A viruses are highly contagious and cause acute respiratory illness, spreading rapidly through populations, particularly during seasonal outbreaks². 

Therefore, understanding the scale, transmission dynamics, and subtypes of influenza A is essential, as it is the only type of influenza virus known to cause global pandemics. This article will explore the influenza A symptoms in adults as well as children, setting the stage for further discussion on its diagnosis, treatment, and prevention strategies. 

Causes of Influenza A

Influenza type A is caused by the influenza A virus, which is a single-stranded RNA virus. It infects the cells lining the respiratory tract and triggers an immune response that leads to the flu².  

Influenza A viruses are classified into subtypes based on the combination of their proteins (hemagglutinin [HA] and neuraminidase [NA]). Currently, the two main influenza A subtypes circulating in humans are A(H1N1) and A(H3N2)^1,2. 

Influenza A is highly contagious. This virus primarily spreads through respiratory droplets released during coughing, sneezing, or talking, and can also be transmitted by touching surfaces that already have been exposed to the virus. One of the key reasons influenza A is so widespread and potentially severe is its ability to undergo frequent genetic changes³: 

• Antigenic drift refers to small and gradual changes in the virus’s surface proteins, allowing it to escape the immune system of humans and cause seasonal outbreaks.  
• Antigenic shift refers to an abrupt and major genetic change that can lead to new viral subtypes with pandemic potential³. 

Common Symptoms of Influenza A

Influenza A typically presents with a sudden onset of symptoms, often making individuals feel acutely unwell within a short period. The most common influenza A symptoms in adults and children include⁴: 

• Chills (during early stage) 
• Sweating 
• High fever 
• Muscle aches (myalgia) 
• Headache 
• Extreme tiredness  
• Dry cough 
• Sore throat 
• Nasal congestion or a runny nose 
• Chest discomfort/shortness of breath (in severe cases) 

In addition to these respiratory symptoms, gastrointestinal symptoms such as nausea, vomiting, and diarrhoea may also occur, especially in children, though they are not always present⁴. 

Note: Although often confused with the common cold, influenza (flu) is generally more severe and comes on more suddenly⁵. Therefore, accurate diagnosis is important for proper management. 

Risk Factors for Influenza A

Certain groups of people are more likely to develop serious complications from influenza A. These include: 

• Older adults (65 years and above): Their immune systems might make it harder to fight off infections¹. 
• Young children (especially under 5 years): Their immune systems are still developing⁶. 
• Pregnant women: Pregnancy can alter the immune system, making expectant mothers more vulnerable to flu-related complications⁶.
• Other illnesses: People with chronic health conditions, such as asthma, diabetes, heart disease, kidney disease, or lung disorders that may experience more severe complications¹. 
• Individuals with weakened immune systems: They may not be able to fight the virus effectively (such as those undergoing chemotherapy or living with HIV/AIDS)⁶.
• Healthcare workers and caregivers: They experience a risk of frequent exposure to sick individuals¹. 

How Is Influenza A Diagnosed?

The initial diagnosis of influenza A usually begins with a clinical evaluation, where doctors assess the patient’s symptoms and medical history⁷. 

To confirm influenza A, the following diagnostic tests may be used: 

• Rapid Influenza Diagnostic Tests (RIDTs): Detect flu antigens from a nasal or throat swab but have low to moderate sensitivity, especially in adults, increasing the risk of false negatives if done later in the illness⁸.
• Rapid Molecular Assays: These nucleic acid amplification tests (NAATs) detect influenza viral RNA with higher sensitivity and specificity than RIDTs, delivering results faster⁴. 
• Reverse Transcription Polymerase Chain Reaction (RT-PCR): A highly accurate test that can identify and differentiate flu types. It is often used in hospitals but may take a few hours to process⁸. 
• Viral Culture: Grows the virus in a lab for detailed analysis. This test is slow and not used for immediate diagnosis⁷. 

Note: If you test influenza A positive, it means the virus has been detected in your body, and you may need rest, supportive care, or antiviral medications depending on symptom severity. 

Treatment Options for Influenza A

Most healthy people with mild flu symptoms recover with rest, fluids, and fever-reducing medicines. However, treatment may be necessary if influenza A symptoms in adults (and children) worsen or do not improve. 

1. Antiviral Medications

• Common antivirals such as the following may help reduce the duration (or severity) of symptoms (if started within 24 to 48 hours of symptom onset) and under medical supervision⁸. 
  • Oseltamivir 
  • Zanamivir 
  • Peramivir 
  • Baloxavir marboxil 

Note: They are usually not needed for mild cases in healthy people⁷. However, patients with mild cases in high-risk group (e.g., elderly, pregnant, chronic illness) may warrant antivirals. 

2. Supportive Care 

• Adequate rest and hydration are helpful¹. 
• Fever and pain-relieving medications such as the following can be taken: 
  • Acetaminophen (paracetamol) 
  • Ibuprofen  
  • Aspirin

Note: Aspirin should be avoided in children due to the risk of Reye’s syndrome⁹. It’s ideal to consult a doctor before. 

• Watching out for warning signs, such as trouble breathing or a high, persistent fever, is important. 

Preventing Influenza A

Being influenza A positive indicates an active infection that can be contagious, so isolation and proper hygiene are important to prevent spreading the virus to others. Here are effective ways to protect yourself and others: 

• Get the Flu Vaccine: Annual vaccination is the best way to safeguard yourself against influenza A infection and its complications¹. 
• Practice Good Hand Hygiene: Wash hands frequently with soap and water or use alcohol-based hand sanitiser. Also, avoid touching your eyes, nose, or mouth, as germs can enter your body this way^1,9. 
• Avoid Close Contact: Try to stay away from people who are sick. Also, if you are sick, keep your distance from others to protect them from getting infected. Physical distancing lowers the risk of spreading respiratory viruses^1,9. 
• Cover Your Mouth/Nose: Make sure to cover your mouth and nose with a tissue when coughing or sneezing to avoid spreading the infection. Also, wearing a mask can further reduce virus spread by blocking infectious droplets, protecting both you and others^1,9. 
• Clean Regularly: Regularly clean commonly touched surfaces like doorknobs, countertops, handrails, and phones to reduce the chance of virus transmission⁹. 
• Improve Air Quality: Increase ventilation by bringing in fresh outdoor air, using air purifiers, or spending time outdoors to reduce airborne virus particles⁹. 
• Maintain Healthy Habits: Support your immune system by getting enough sleep, staying physically active, managing stress, drinking plenty of fluids, and eating nutritious foods⁹. 

Also Read: Everything To Know About the Influenza Vaccine & Its Importance 

When to See a Doctor

While most people recover from influenza type A within a few days to two weeks, it’s important to seek medical care if symptoms worsen or new complications arise. You should see a doctor as soon as possible if you experience¹⁰: 

• Trouble breathing  
• Chest pain or pressure 
• Sudden dizziness or confusion 
• High fever (≥39°C or 102.2°F) (persisting for more than 3 days or not responding to medication) 
• Bluish lips or face 
• Severe muscle pain, weakness, or difficulty walking 
• Seizures 
• Decreased or no urine output 
• Worsening of chronic conditions (such as asthma) 

Note: If you observe that your symptoms improve for a while but then return with fever and worse cough, it may indicate a secondary bacterial infection. Therefore, it is very important to contact a doctor in such cases. 

High-risk individuals should be especially vigilant and contact a doctor early. Complications to watch for in influenza type A include³: 

• Pneumonia (viral or bacterial) 
• Sinus and ear infections 
• Myocarditis (heart muscle inflammation)  
• Encephalitis (brain inflammation) 
• Myositis or rhabdomyolysis (muscle inflammation or breakdown with severe pain, and risk of kidney damage)  
• Sepsis (a life-threatening response to infection) 
• Worsening of existing medical conditions 

Also Read: What is Influenza (Flu) & Its Types

Conclusion

Understanding and effectively managing influenza A is vital to reducing its impact on individuals and communities. While many cases are mild, the flu can lead to serious complications, especially in high-risk groups. Therefore, timely diagnosis, appropriate treatment, and preventive measures like vaccination, hand hygiene, and staying home when sick can help avoid the spread of the virus significantly.  

Additionally, raising awareness about symptoms, risk factors, and when to seek medical help empowers people to take proactive steps, ensures better health outcomes, and avoids severe illness or death from flu-related complications.

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

Frequently Asked Questions (FAQs)

References

1. World Health Organization. Influenza – India [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) 
2. Belshan MA, Knoop FC, Huggett KN. Influenza A. Reference Module in Biomedical Sciences. 2014:B978-0-12-801238-3.05048-0. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC7157457/ 
3. Moghadami M. A Narrative Review of Influenza: A Seasonal and Pandemic Disease. Iran J Med Sci. 2017 Jan;42(1):2-13. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5337761/ 
4. Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016 Dec 7;355:i6258. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5141587/ 
5. Centers for Disease Control and Prevention. Cold Versus Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/about/coldflu.html 
6. 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/ 
7. Gaitonde DY, Moore FC, Morgan MK. Influenza: Diagnosis and Treatment. Am Fam Physician. 2019 Dec 15;100(12):751-758. Available from: https://pubmed.ncbi.nlm.nih.gov/31845781/ 
8. Vemula SV, Zhao J, Liu J, Wang X, Biswas S, Hewlett I. Current Approaches for Diagnosis of Influenza Virus Infections in Humans. Viruses. 2016 Apr 12;8(4):96. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4848591/ 
9. 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 
10. Centers for Disease Control and Prevention. Signs and Symptoms of Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/signs-symptoms/index.htm 

Disclaimer: The information provided here is for educational/awareness purposes only and is not intended to be a substitute for medical treatment by a healthcare professional and should not be relied upon to diagnose or treat any medical condition. The reader should consult a registered medical practitioner to determine the appropriateness of the information and before consuming any medication. PharmEasy does not provide any guarantee or warranty (express or implied) regarding the accuracy, adequacy, completeness, legality, reliability or usefulness of the information; and disclaims any liability arising thereof.