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. 

Influenza A Subtypes

Influenza A viruses are divided into subtypes based on their surface proteins. Different subtypes can vary in how easily they spread, how severe the illness is, and who is most at risk. The three most discussed subtypes are H1N1, H3N2, and H5N1³. 

H1N1

• A seasonal human influenza strain that circulates regularly.
• Responsible for the 2009 influenza pandemic (often referred to as “swine flu”).
• Now established as one of the common seasonal flu strains.

H3N2

• A common seasonal strain that circulates in humans.
• Often associated with more severe illness, particularly in older adults.
• Known for frequent antigenic drift, meaning it changes often, which can reduce vaccine match in some seasons.

H5N1

• An avian influenza (bird flu) virus that primarily affects birds.
• Human infection is rare, but when it does occur, it can be severe.
• Importantly, it does not usually spread easily from person to person, which limits its ability to cause widespread human outbreaks under normal circumstances.

In summary, different influenza A subtypes differ in severity, transmission, and risk. Seasonal strains such as H1N1 and H3N2 spread readily among people, while H5N1 is rare in humans but potentially serious.

Current Circulating Strains

Influenza is a seasonal illness, and the strains that circulate can vary from one season and region to another. This is part of why annual vaccination is recommended¹³.

• In recent seasons in India, surveillance data has generally shown alternating dominance between the H1N1 and H3N2 subtypes, with one or the other being more prominent in a given season.
• These patterns can shift, so the exact dominant strain in any particular season is not fixed.
• Updated strain information is monitored and reported annually by public health authorities, who track circulating viruses to guide vaccine composition and public health planning.

For the most current information on circulating strains, it is best to refer to updates from recognised public health authorities rather than relying on fixed assumptions, as the picture changes each season.

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. 

The influenza vaccine shot can easily be taken in the arm, unlike other injections, that typically need to be taken in the buttock region¹².

Dr. Nayana Shetty, MBBS, MD

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. 

Also Read: What Is Hantavirus? Symptoms, Causes, & Effective Prevention

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

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

3. Oseltamivir Treatment Window

Timing is one of the most important factors in antiviral treatment for influenza A. Antivirals such as oseltamivir work best when started early in the course of illness¹⁴.

• Antiviral treatment is most effective when started within 48 hours of symptom onset, as this is when it is most likely to reduce symptom duration and may reduce the risk of certain complications.  
• If started later than 48 hours, the benefit is generally reduced. However, it may still be considered in severe cases or in high-risk individuals, under medical guidance.
• Because of this narrow window, it is important to seek medical advice early if influenza is suspected, particularly for those at higher risk of complications.

Early consultation allows a doctor to assess whether antiviral treatment is appropriate and to start it within the most effective timeframe.

High-Risk Group Prioritisation for Treatment

While many healthy people recover from influenza A with rest and supportive care, antiviral treatment is prioritised for those at higher risk of serious complications⁷. These groups include:

• Elderly individuals (65 years and above).
• Pregnant women.
• Young children.
• People with chronic illnesses, such as asthma, diabetes, heart disease, or lung disease.
• Immunocompromised individuals, such as those undergoing chemotherapy or living with HIV/AIDS.

For these high-risk groups, a few important points apply:

• They may be given antiviral treatment even if their symptoms are mild, because their risk of complications is higher.
• Starting treatment early tends to improve outcomes in these individuals.

If you or someone you care for falls into a high-risk group and develops flu-like symptoms, it is best to contact a doctor promptly rather than waiting to see if symptoms worsen. 

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. 

Since the influenza virus tends to change and evolve each year, a new batch of vaccines is produced each year. Thus, each year when the vaccine is given, it actually protects against the virus currently spreading infection. It is like a new virus vaccine each year¹¹.

Dr. Nayana Shetty, MBBS, MD

Post-Exposure Prophylaxis (Prevention After Exposure)

Post-exposure prophylaxis (PEP) means taking preventive medication after being exposed to an infection, to reduce the chance of falling ill¹⁵.

In the context of influenza A, antiviral medication such as oseltamivir may sometimes be considered after close contact with a confirmed influenza case, particularly for those at higher risk of complications.

Examples of situations where PEP may be considered include:

• Household exposure, such as living with someone who has confirmed influenza.
• Healthcare exposure, such as a healthcare worker or caregiver in close contact with a person with influenza.

It is important to note that PEP is used selectively, based on clinical judgement. A doctor will weigh factors such as the person’s risk level, the nature of the exposure, and timing before recommending it. PEP is not a substitute for vaccination and good hygiene practices.

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

When to Return to School or Work?

Knowing when it is safe to return to school or work after influenza A helps protect others and reduce the spread of the virus in community settings⁹.

As a general guide, it is usually appropriate to return when:

• You have been fever-free for at least 24 hours without the use of fever-reducing medication (such as paracetamol), and you feel well enough to resume normal activities. 
• Your overall symptoms are improving.

A few additional points to keep in mind:

• Children may require stricter monitoring before returning, as they can sometimes remain contagious for longer than adults and may need closer observation.
• Even after returning, continuing good hygiene practices, such as handwashing and covering coughs and sneezes, helps prevent transmission to others in shared environments.

Preventing the spread of influenza in schools and workplaces benefits everyone, particularly those who may be at higher risk of complications. 

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

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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 
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11. Mäkelä MJ, Puhakka T, Ruuskanen O, Leinonen M, Saikku P, Kimpimäki M, Blomqvist S, Hyypiä T, Arstila P. Viruses and bacteria in the etiology of the common cold. J Clin Microbiol. 1998 Feb;36(2):539-42. doi: 10.1128/JCM.36.2.539-542.1998. PMID: 9466772; PMCID: PMC104573. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC104573/
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14. Sur M, Lopez MJ, Patel P, et al. Oseltamivir. [Updated 2024 Feb 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539909/
15. National Institutes of Health. Post-exposure prophylaxis (PEP) [Internet]. Bethesda (MD): NIH HIVinfo; [cited 2026 Jun 25]. Available from: https://hivinfo.nih.gov/understanding-hiv/fact-sheets/post-exposure-prophylaxis-pep

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