What is seasonal flu? Causes, symptoms and how to cope with the disease

Seasonal influenza epidemics represent a recurrent public health issue that typically emerges during colder months, affecting millions of individuals annually. Although influenza is often perceived as a common illness, inadequate prevention and improper treatment can result in serious complications, potentially endangering the patient's life.

So, what exactly is a seasonal influenza epidemic? What are its causes, clinical manifestations, and effective preventive measures? These questions will be explored in detail in the following article.

Dr. Tran Huynh Tan – Regional Medical Manager, Ho Chi Minh City Region 1, VNVC Vaccination Center System, stated: “Influenza is a highly contagious disease that spreads widely across the globe, with annual infection rates ranging from 5–10% in adults and 20–30% in children (1). Each year, approximately one billion cases of seasonal influenza are recorded worldwide, including 3 to 5 million severe cases, leading to an estimated 290,000 to 650,000 deaths due to respiratory complications. Influenza vaccination is an effective solution to reduce the incidence of the disease by 40–60%, helping to protect public health and alleviate the burden of the epidemic.”

What is an influenza epidemic?

An influenza epidemic refers to a phenomenon in which the influenza virus spreads widely within a community, causing a sudden outbreak with a high number of infections in a specific region or across an entire country. Influenza is an infectious disease caused by the influenza virus, primarily affecting the respiratory system. The virus can be transmitted through respiratory droplets when an infected person coughs or sneezes, or via direct contact with contaminated surfaces.

Common symptoms of influenza include high fever, cough, sore throat, fatigue, and myalgia (muscle pain). In severe cases, it may lead to serious complications such as pneumonia and respiratory failure, particularly among vulnerable populations such as young children, the elderly, and individuals with underlying medical conditions. Influenza outbreaks often occur during the winter season or when there is a shift in the circulating viral strains.

Historically, influenza pandemics have claimed the lives of millions. One of the most devastating influenza pandemics was the Spanish Flu of 1918. Approximately 500 million individuals—equivalent to one-third of the global population at that time—were infected. The estimated death toll ranged from 50 million to possibly 100 million, according to some experts. In the United States alone, about 675,000 people died due to this pandemic.

Subsequently, the Asian Flu pandemic of 1957–1958 infected around 500 million people globally and caused at least 1 million deaths. Not long after, the Hong Kong Flu pandemic (1968–1969) also affected approximately 500 million people, resulting in an estimated 1 to 4 million deaths. The Russian Flu pandemic (1977–1979), although lacking precise data on the number of infections, was estimated to have caused approximately 700,000 deaths—an alarming figure.

An influenza epidemic An influenza pandemic occurs when the influenza virus spreads rapidly through a community, creating an outbreak with a large number of people infected, affecting an area or the entire country.

Causes of Influenza Outbreaks

The primary cause of influenza outbreaks is the influenza virus—a virus that mutates easily and can spread through various transmission routes. Influenza viruses belong to the Orthomyxoviridae family and include several types: A, B, C, and D.

Among these, types A and B are the main culprits behind flu outbreaks. Influenza A viruses often cause large-scale epidemics and can rapidly mutate their genetic material, leading to new variants with stronger transmissibility. Influenza A can also be transmitted from animals to humans, especially from poultry, potentially triggering dangerous pandemics. Influenza B viruses, though less prone to mutation, can still cause smaller outbreaks that significantly affect public health. Types C and D are less common and rarely cause large-scale epidemics.

The flu virus spreads primarily through respiratory droplets released when an infected person coughs, sneezes, or talks. These droplets can travel through the air and enter a healthy person’s body via the nose, mouth, or eyes. Additionally, the virus can spread through direct contact with an infected person or indirectly via contaminated surfaces—such as doorknobs, keyboards, or phones—followed by touching the face.

Environmental and behavioral factors also play a significant role in influenza transmission. During the winter season, cold and dry weather leads people to spend more time indoors in crowded and poorly ventilated spaces, creating ideal conditions for the virus to spread rapidly. High-density environments such as schools, hospitals, and workplaces are especially vulnerable to outbreaks due to close contact between individuals.

People with weakened immune systems—including children, the elderly, pregnant women, and those with underlying health conditions like heart disease, diabetes, or chronic lung disease—are more susceptible to infection and at greater risk of developing severe complications. This increases the likelihood of virus transmission within the community, particularly because these vulnerable individuals may exhibit mild or delayed symptoms.

The influenza virus has the ability to alter its genetic structure, resulting in new viral strains through processes such as antigenic drift or genetic reassortment. These changes allow the virus to evade the human immune system, enhancing its capacity to spread and cause illness. This is why influenza outbreaks occur regularly and may become increasingly severe over time.

Signs of a Seasonal Influenza Outbreak

A seasonal influenza outbreak is typically identified by a sudden increase in flu cases within a community. Key indicators include:

A surge in flu cases within the community: One of the clearest signs is a sharp rise in the number of infections within a specific area. This is often observed through healthcare centers, hospitals, and clinics that report a spike in flu-related visits over a short period.
High infection rates among vulnerable populations: Groups such as children, the elderly, and immunocompromised individuals are more prone to infection. A rise in cases among these groups often signals the onset of a flu outbreak.
Increased number of flu cases with complications: During an outbreak, there is typically a rise in hospital admissions due to flu-related complications such as pneumonia, respiratory failure, or exacerbation of underlying conditions like heart or lung disease, or diabetes. These complications tend to be more severe in individuals with poor health.
Widespread appearance of common flu symptoms: As the flu virus spreads, typical symptoms—including high fever, cough, sore throat, muscle aches, fatigue, and nasal congestion—become prevalent across the population. These symptoms are especially noticeable in densely populated areas like schools, hospitals, and offices.
Rapid transmission within the community: Seasonal influenza spreads easily through airborne droplets produced when an infected person coughs, sneezes, or even talks. Without effective prevention measures, outbreaks can escalate quickly in crowded public settings.
Emergence of new influenza strains: Each year, influenza viruses may undergo genetic changes, resulting in new strains to which the population has little or no immunity. The appearance and rapid spread of such a new strain are often signs of a potentially severe seasonal outbreak.

An influenza epidemic The hallmark of a seasonal flu outbreak is a rapid increase in the number of cases and severe complications over a short period of time.

Symptoms of Seasonal Influenza Infection

Seasonal influenza can present with a wide range of symptoms, varying from mild to severe, depending on the individual and the degree of viral impact. The following are common symptoms associated with influenza:

1. Mild Symptoms

In mild cases, individuals typically experience basic flu symptoms and often recover within a few days without the need for specialized treatment. Mild symptoms may include:

Low-grade fever or chills: A slight elevation in body temperature accompanied by a feeling of coldness.
Dry or productive cough: Coughing is the most frequent symptom and may persist for several days.
Runny or congested nose: Leads to nasal blockage and difficulty breathing.
Sore throat: A scratchy or painful sensation when swallowing food or liquids.
Fatigue and sluggishness: The patient may feel drained and unwilling to engage in normal activities.
Mild muscle aches or headaches: Generalized muscle discomfort and occasional mild headaches are common.

2. Severe Symptoms

In certain cases, influenza may progress to a more serious form, leading to potentially life-threatening complications—especially among individuals with weakened immune systems or underlying health conditions. Severe symptoms include:

High fever that does not subside with medication
Shortness of breath, rapid breathing, or irregular breathing
Little to no urination
Persistent chest or abdominal pain
Frequent dizziness or a near-fainting sensation
Confusion or inability to concentrate
Prolonged muscle pain or generalized weakness

High-Risk Populations for Seasonal Influenza Complications

Although influenza can affect anyone, certain groups are at higher risk of developing severe illness or complications due to underlying vulnerabilities in health or immune function. These groups include:

Children under 5 years of age: Young children have immature immune systems, making them more susceptible to severe flu outcomes, especially those under 2 years old.
Older adults (aged 65 and above): Aging is associated with a natural decline in immune function, increasing the risk of complications such as pneumonia and respiratory failure.
Individuals with chronic medical conditions: Patients with cardiovascular disease, asthma, diabetes, chronic kidney disease, or chronic pulmonary conditions are more vulnerable to serious illness and complications.
Pregnant women: Pregnancy induces immune system changes that heighten vulnerability to infections, including seasonal influenza, posing risks to both mother and fetus.
People with weakened immune systems: Individuals undergoing cancer treatment, taking immunosuppressive medications, or living with autoimmune disorders are at greater risk of contracting influenza and experiencing severe complications.
Healthcare workers and caregivers: Due to frequent close contact with patients, medical personnel and those working in high-density environments face a significantly higher risk of influenza infection.

An influenza epidemic Children under 5 years old have an incomplete immune system so they are easily attacked by the flu virus and get sick.

How Dangerous Is Influenza?

The influenza virus spreads rapidly through airborne droplets and direct contact, making flu outbreaks highly contagious and easily transmissible within communities. This widespread transmission increases the burden on healthcare systems, particularly during flu season, when the number of infected individuals rises sharply and often leads to hospital overcrowding.

Influenza can cause serious health complications, especially in vulnerable populations such as young children, the elderly, pregnant women, and individuals with underlying medical conditions like cardiovascular disease, diabetes, or chronic respiratory illnesses.

Although the majority of flu cases resolve within a few days to two weeks, some individuals may develop severe complications that can become life-threatening. These complications include pneumonia and may also trigger or worsen other conditions such as sinusitis, otitis media, arthritis, or exacerbate chronic diseases like diabetes, immune deficiencies, and cardiovascular conditions such as hypertension, coronary artery disease, and heart failure.

Pregnant women are particularly at risk if infected with influenza during the first trimester. This critical developmental period for the fetus involves the formation of major organs, and flu infection can increase the risk of congenital abnormalities, miscarriage, or stillbirth.

Beyond its impact on individual health, influenza outbreaks can have significant economic and societal consequences. Millions of people may be forced to take time off from work or school, leading to decreased productivity and placing additional strain on national economies. Therefore, preventing influenza through vaccination and proper hygiene practices is essential to mitigate these widespread negative effects.

When Should You See a Doctor?

If an individual with the flu experiences any of the following serious symptoms, they should be taken to the emergency room immediately for medical evaluation and timely intervention:

High fever (above 40°C / 103°F)
Difficulty breathing or rapid breathing
Little or no urination
Persistent chest or abdominal pain that does not subside
Continuous dizziness or feeling faint
Confusion or inability to concentrate
Severe muscle pain or prolonged muscle weakness
Sudden seizures
Pale, bluish, or grayish skin, lips, or fingernails—possible signs of oxygen deficiency in the blood or tissues
Fever or cough that initially improves but then worsens
Worsening of existing health conditions Flu (Influenza)

An influenza epidemic When having flu, if the patient has symptoms of high fever, difficulty breathing, persistent chest or abdominal pain... they need to be taken to the hospital immediately for timely examination and treatment by a doctor.

How to Treat Influenza

1. Medical Treatment for Influenza

To treat influenza, doctors may prescribe antiviral medications that help reduce the severity of symptoms and shorten recovery time. These include:

Oseltamivir phosphate (Tamiflu®): Available in capsule or liquid form, this medication is commonly used to treat and prevent influenza.
Zanamivir (Relenza®): Administered by inhalation through a special device, Zanamivir is typically used for a few days. However, it is not recommended for individuals with respiratory conditions such as asthma or COPD.
Peramivir (Rapidvap®): Given as a single intravenous (IV) dose, this medication is used in more severe cases and has shown effective results with just one administration.
Baloxavir marboxil (Xofluza®): Taken as a single oral dose in tablet or liquid form. However, it is not recommended during pregnancy, breastfeeding, or in individuals with severe health conditions.

In addition to prescription antivirals, patients may also manage flu symptoms at home with over-the-counter (OTC) medications such as:

Acetaminophen (Tylenol®) or NSAIDs (Advil®, Motrin®, Aleve®): Help reduce fever, relieve headaches, and ease body aches.
Decongestants like phenylephrine or pseudoephedrine (available in sprays or oral tablets): Help relieve nasal congestion and runny nose for easier breathing.
Dextromethorphan: An effective cough suppressant that helps soothe persistent and irritating coughs.
Guaifenesin: An expectorant that loosens mucus in the lungs, clearing the airways and improving breathing.

These medications can alleviate symptoms and limit the progression of the influenza virus. However, their use should always follow the instructions and advice of a healthcare provider for maximum effectiveness and safety.

2. Home Remedies for Influenza

Home treatment for influenza focuses on relieving symptoms and supporting the body's natural recovery process. Some recommended approaches include:

Adequate Rest: Rest is crucial for allowing the body to fight off the flu virus. Patients should avoid strenuous activities and give their body time to heal.
Stay Hydrated: Flu can cause dehydration, so it’s important to drink plenty of fluids such as water, fruit juices, or warm broths to maintain hydration and ease symptoms like dry throat or nasal congestion.
Steam Inhalation: Breathing in warm steam from a bowl of hot water or using a humidifier can help soothe the airways and relieve nasal congestion.
Traditional Remedies: Some natural methods like drinking ginger tea with honey, gargling with warm salt water, or taking steam baths can help alleviate symptoms and boost the immune system.

An influenza epidemic To treat the flu, patients should drink plenty of water to balance the body's water and reduce symptoms of nasal congestion or dry throat.

Effective Ways to Prevent Influenza Epidemics

1. Get an Annual Flu Vaccine

Getting vaccinated against the flu is currently the most effective, safe, and cost-efficient method of flu prevention. The flu vaccine helps the body produce antibodies that protect against the flu and the dangerous complications it may cause. It is estimated that during the 2019–2020 flu season, flu vaccination helped prevent about 7 million flu cases, reduced 3 million doctor visits, 100,000 hospitalizations, and saved approximately 7,000 lives.

Flu vaccines need to be administered annually because immunity decreases over time. Moreover, flu viruses frequently mutate and new strains appear, so the vaccine composition must be updated each year to match the circulating strains during the flu season. Therefore, regular flu vaccination is an important way to protect your health and reduce the impact of a flu epidemic.

2. Maintain Personal Hygiene

Maintaining personal hygiene is one of the most important measures to effectively prevent flu outbreaks. Below are some ways to practice good hygiene to reduce your risk of catching the flu:

Wash hands frequently: Use soap and clean water for at least 20 seconds, especially after coughing, sneezing, using the restroom, or touching public surfaces. If soap and water are unavailable, use a hand sanitizer containing at least 60% alcohol.
Use tissues when coughing or sneezing: To prevent the spread of infection, use a tissue to cover your mouth and nose when coughing or sneezing. Dispose of the tissue in a trash bin and wash your hands immediately. If a tissue is not available, cough or sneeze into your elbow to limit the spread of germs.
Avoid touching eyes, nose, and mouth: Flu viruses can enter your body through the eyes, nose, and mouth after touching contaminated surfaces.
Keep living and working spaces clean: Regularly disinfect frequently touched surfaces such as doorknobs, keyboards, phones, and shared objects to prevent the spread of viruses.
Wear a mask when necessary: If you have flu symptoms or are feeling unwell, wear a mask to limit the spread of the virus when coughing, sneezing, or talking.

An influenza epidemic To prevent the spread of flu, wearing a mask when going out is an effective measure to help protect yourself and the community from the risk of virus infection.

3. Strengthening the Immune System

Enhancing the immune system is one of the most effective ways to prevent influenza outbreaks and other infectious diseases. Below are several methods to naturally boost your body’s defenses:

Maintain a balanced and nutritious diet: A well-rounded diet rich in essential nutrients helps the immune system function more efficiently. Incorporate plenty of green vegetables, fruits, and foods high in vitamin C (such as oranges, tangerines, red bell peppers, strawberries), vitamin D (from sunlight and foods like salmon, eggs, and dairy), and zinc (found in cashews, chicken, and whole grains).
Stay hydrated: Adequate water intake is vital for overall body functions, including immunity. Water assists in detoxification and supports the optimal performance of immune cells.
Get enough sleep: Sleep plays a critical role in immune recovery and resistance. Adults should aim for at least 7–8 hours of sleep each night to maintain a strong immune system.
Exercise regularly: Physical activities such as walking, jogging, yoga, or light workouts promote circulation and strengthen the immune system. Regular exercise also helps reduce stress, which in turn lowers the risk of illness.
Manage stress effectively: Chronic stress can weaken immune defenses, making the body more susceptible to infections. Techniques like meditation, yoga, or deep breathing can help manage stress and enhance immunity.
Avoid harmful exposures: Limit contact with cigarette smoke, air pollution, and other harmful substances that may suppress immune function. Maintaining a healthy living environment supports optimal immune performance.
Consider dietary supplements (if necessary): If your diet lacks essential nutrients, consult a healthcare professional about supplementing with vitamins and minerals such as vitamin C, D, or zinc.

Recent Influenza Outbreaks

In recent times, influenza has experienced significant surges globally, raising major public health concerns. One of the most notable recent influenza pandemics was the H1N1 flu outbreak in 2009, also known as the swine flu, which resulted in thousands of deaths worldwide.

According to the World Health Organization (WHO), there were 1,632,258 confirmed H1N1 cases with 18,036 reported deaths. However, the U.S. Centers for Disease Control and Prevention (CDC) estimates that the true death toll may have reached up to 284,000 due to underreporting and lack of verification.

Between 2024 and 2025, CDC estimates show that from the beginning of the 2024 flu season to February 2025, the U.S. recorded at least 24 million flu cases, with approximately 310,000 hospitalizations and 13,000 deaths. Alarmingly, at least 57 of the deaths were pediatric cases. As of February 7, 2025, the number of outpatient visits for influenza had reached the highest level in 15 years.

In Japan, according to the National Institute of Infectious Diseases (NIID), between September 2, 2024, and January 26, 2025, approximately 9.523 million influenza cases were recorded. Notably, during the week of December 23–29, 2024, around 5,000 healthcare facilities reported a total of 318,000 flu cases—three times higher than the same period the previous year.

This was the first time weekly flu cases exceeded 300,000, breaking the record since Japan adopted its current surveillance system in 1999. The average number of flu cases per facility reached 64 per week—double the national alert threshold of 30 cases/week set by the Japanese government.

How to Care for a Patient with Influenza

Proper care for individuals suffering from influenza is crucial to support a swift recovery and prevent complications. Below are key recommendations for caring for a flu patient:

Ensure Adequate Rest: Rest is essential for recovery. Allowing the patient to rest helps the immune system function more effectively in combating the influenza virus.
Maintain Proper Hydration: Influenza often leads to dehydration due to fever, sweating, or vomiting. Patients should be encouraged to drink plenty of fluids such as water, fruit juices, or clear broths to replenish lost fluids and prevent dehydration.
Use Symptom-Relief Medications Appropriately: Over-the-counter medications such as paracetamol (acetaminophen) or ibuprofen can help alleviate fever and muscle aches. However, always consult a healthcare professional before administering any medication, especially to children.
Monitor Symptoms Closely: Seek immediate medical attention if the patient experiences unusual symptoms such as difficulty breathing, chest pain, or a worsening condition. Timely intervention can prevent severe complications.
Maintain Good Hygiene: Encourage frequent hand washing and the use of face masks when the patient coughs or sneezes to reduce the risk of spreading the virus. Regularly disinfect high-touch surfaces and maintain a clean environment around the patient.
Encourage Light, Nutritious Meals: Even if the patient has a reduced appetite, it is important to consume easily digestible, nutrient-rich foods. Soft foods such as porridge, soups, or fruits can help provide essential energy and support the healing process.
Create a Comfortable Recovery Environment: Ensure that the patient’s room is well-ventilated, quiet, and comfortable. If possible, keep the room at a cool temperature to ease discomfort and help reduce fever.

An influenza epidemic When having the flu, patients need to take medication as directed by their doctor and ensure adequate rest so that the body can recover quickly.

Seasonal influenza is a common infectious disease that can have serious impacts on public health. Although most people infected with the flu can recover within a short period, improper treatment may lead to dangerous complications. Therefore, getting an annual flu vaccination and maintaining good personal hygiene are effective preventive measures that help reduce the risk of infection and protect health throughout the flu season.

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