Influenza (Flu): Symptoms, Causes, and Treatment Options

What Is Influenza?

Influenza, commonly known as the flu, is far more than a “bad cold.” It is an acute respiratory infectious disease caused by influenza viruses that poses a serious threat to global public health. Influenza is a highly contagious viral infection that primarily attacks the respiratory system, the nose, throat, bronchial tubes, and lungs. The disease is caused by influenza viruses, which are classified into types A, B, C, and D. Influenza A and B viruses are responsible for the seasonal epidemics that sweep across the globe each year.

Each year, seasonal influenza epidemics sweep across the globe, causing millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths. Unlike the common cold, which typically resolves with rest and fluids, influenza can lead to severe complications, including pneumonia, respiratory failure, and death, particularly among vulnerable populations.

The 1918 Spanish flu pandemic, the 1950-1951 epidemic that prompted extensive etiological study and virus isolation efforts, and the more recent 2009 H1N1 pandemic serve as stark reminders of influenza’s devastating potential. Even in non-pandemic years, seasonal influenza imposes a substantial burden on healthcare systems, disrupts workplaces and schools, and causes significant morbidity and mortality among high-risk groups including the elderly, infants and young children, pregnant women, and patients with chronic underlying diseases.

Types and Classifications of Influenza Viruses

Influenza viruses belong to the Orthomyxoviridae family and are classified into four types, with types A and B being the most clinically significant.

Influenza A

Influenza A viruses are the most virulent and are responsible for the majority of seasonal epidemics as well as all documented pandemics. These viruses are further subtyped based on the combinations of two surface proteins: hemagglutinin (H) and neuraminidase (N).

• Hemagglutinin (HA): There are 18 different HA subtypes (H1-H18). HA facilitates viral attachment to host cells, initiating infection. The hemagglutination-inhibition (HI) assay is a classical laboratory procedure used to identify the HA subtype of an unknown isolate or the HA subtype specificity of antibodies to influenza virus.
• Neuraminidase (NA): There are 11 different NA subtypes (N1-N11). NA enables the release of newly formed viral particles from infected host cells, allowing the infection to spread.

Common seasonal influenza A subtypes include H1N1 (which caused the 2009 pandemic and continues to circulate) and H3N2. A 2017 pharmacoepidemiological study detected pandemic strain swine influenza A/H1N1 in 28.8% of laboratory-confirmed cases and A/H3N2 in 6.3% of cases during specific seasons .

Influenza B

Influenza B viruses are less genetically diverse than influenza A and are not classified into subtypes. Instead, they are divided into two main lineages: Victoria and Yamagata. Influenza B contributes to seasonal epidemics and can cause severe illness, though it does not cause pandemics. A recent epidemiological study conducted between 2019 and 2024 found that influenza A remained the predominant subtype in hospital data, though influenza B accounted for a substantial proportion of cases in national surveillance data .

Influenza C and D

Influenza C viruses cause mild respiratory illness and are not associated with epidemics. Influenza D primarily affects cattle and is not known to cause illness in humans.

Laboratory Identification

The classification of influenza viruses relies on laboratory techniques including the hemagglutination-inhibition (HI) assay, which is a relatively inexpensive procedure utilizing standard laboratory equipment that can be completed within several hours . However, this method requires extensive library of reference reagents for identifying antigenically distinct influenza viruses or antibody specificities from multiple lineages.

Characteristic	Influenza A	Influenza B	Influenza C
Host Range	Humans, birds, pigs, horses	Humans primarily	Humans, pigs
Subtypes	H1N1, H3N2, others	Victoria, Yamagata lineages	Not subtyped
Pandemic Potential	Yes	No	No
Severity	Moderate to severe	Moderate	Mild
Genetic Variability	High (antigenic drift and shift)	Moderate (drift only)	Low

 

Causes and Risk Factors Associated with Influenza

The Viral Etiology

Influenza is caused by exposure to influenza viruses, which are transmitted from person to person primarily through respiratory droplets produced when infected individuals cough, sneeze, or talk. Less commonly, transmission can occur through contact with contaminated surfaces (fomites) followed by touching the mouth, nose, or eyes.

Risk Factors for Severe Influenza

Age-related risk factors:

• Children younger than 2 years old
• Adults older than 65 years

Medical risk factors:

• Chronic somatic diseases
• Endocrine disorders (including diabetes)
• Metabolic disorders, including obesity (body mass index exceeding 30 kg/m²)
• Circulatory system (cardiovascular) diseases
• Respiratory diseases (including chronic obstructive pulmonary disease and asthma)

Physiological risk factors:

• Pregnancy

Other risk factors:

• Immunosuppression (due to medications or medical conditions)
• Residence in long-term care facilities
• Healthcare occupation (increased exposure risk)

Signs and Symptoms of Influenza: Recognizing Influenza

Influenza is classically characterized by the sudden onset of systemic and respiratory symptoms. However, relying solely on clinical presentation to diagnose influenza is challenging, as symptoms can overlap significantly with other respiratory viruses.

Classic Symptoms in Adults

Certain symptoms and signs increase the likelihood of influenza, while others make the diagnosis less likely .

Symptoms that increase likelihood of influenza (positive likelihood ratios):

• In patients aged 60 years or older:

1. Combination of fever, cough, and acute onset: LR = 5.4 (95% CI, 3.8-7.7)
2. Fever and cough: LR = 5.0 (95% CI, 3.5-6.9)
3. Fever alone: LR = 3.8 (95% CI, 2.8-5.0)
4. Malaise: LR = 2.6 (95% CI, 2.2-3.1)
5. Chills: LR = 2.6 (95% CI, 2.0-3.2)

Symptoms that decrease likelihood of influenza (negative likelihood ratios <0.5):

1. Absence of fever: LR = 0.40 (95% CI, 0.25-0.66)
2. Absence of cough: LR = 0.42 (95% CI, 0.31-0.57)
3. Absence of nasal congestion: LR = 0.49 (95% CI, 0.42-0.59)

Symptoms in High-Risk Groups

A 2017 pharmacoepidemiological study found that high-risk groups, including children under 2 years, adults over 65, pregnant women, and individuals with chronic diseases or obesity, exhibited a more severe course of flu-like syndrome than patients outside risk groups . The incidence of complications, particularly pneumonia, was substantially higher in these populations, especially in children under 2 years and patients with endocrine, metabolic, or respiratory diseases.

Summary of Common Influenza Symptoms

Symptom Category	Specific Symptoms
Systemic	Fever (≥37.8°C), chills, malaise, myalgia (muscle aches), headache
Respiratory	Cough (typically dry and non-productive), nasal congestion, sore throat (less common)
Gastrointestinal	Nausea, vomiting, diarrhea (more common in children than adults)
Onset Characteristics	Acute (sudden) onset of symptoms, often described as “hit by a truck”

When to Seek Emergency Medical Attention

Emergency warning signs requiring immediate medical care include:

1. Difficulty breathing or shortness of breath
2. Chest pain or pressure
3. Sudden dizziness or confusion
4. Severe or persistent vomiting
5. Symptoms that improve but then return with fever and worse cough
6. In infants: inability to feed, trouble breathing, no tears when crying, significantly fewer wet diapers

Diagnosis for Influenza

Laboratory Diagnostic Tests

Rapid Influenza Diagnostic Tests (RIDTs):

1. Provide results in 10-30 minutes
2. Detect influenza viral antigens
3. Sensitivity ranges from 50-70% (moderate sensitivity)
4. High specificity (95-99%): positive results are reliable
5. Negative results DO NOT rule out influenza due to limited sensitivity

Molecular Tests (RT-PCR):

1. Gold standard for influenza diagnosis
2. Highly sensitive and specific (detects viral RNA)
3. Can distinguish between influenza A and B and identify subtypes
4. Results typically available in 1-8 hours
5. More expensive than RIDTs

Viral Culture:

1. Results take 3-10 days
2. High specificity but lower sensitivity than PCR
3. Not useful for clinical decision-making; primarily used for surveillance and research

Serologic Testing:

1. Measures antibody response to influenza
2. Requires acute and convalescent samples (2-4 weeks apart)
3. Not useful for acute clinical management; primarily used for research and epidemiologic studies

The hemagglutination-inhibition (HI) assay is a classical laboratory procedure that remains important for subtyping influenza viruses and evaluating antigenic relationships between different isolates. While less technical than molecular tests, the HI assay requires extensive library of reference reagents and is primarily used in reference laboratories.

Treatment Options for Influenza: Antiviral Therapy and Supportive Care

The Importance of Early Treatment

Early antiviral treatment is key to reducing the rate of severe illness and death from influenza. High-risk groups, including the elderly, infants and young children, pregnant women, and patients with chronic underlying diseases, are particularly vulnerable to developing severe illness after infection, which can lead to serious complications and even death.

Antiviral Medications

Neuraminidase Inhibitors:

• Oseltamivir (Tamiflu®): Oral medication approved for treatment of influenza in patients aged 2 weeks and older. For best effect, initiate within 48 hours of symptom onset.
• Zanamivir (Relenza®): Inhaled medication approved for treatment in patients aged 7 years and older. Not recommended for patients with underlying respiratory disease (asthma, COPD).
• Peramivir (Rapivab®): Intravenous medication administered as a single dose; approved for treatment in patients aged 6 months and older.

Cap-Dependent Endonuclease Inhibitor:

• Baloxavir Marboxil (Xofluza®): Oral, single-dose medication approved for treatment in patients aged 5 years and older.

Combination Antiviral Therapy

According to the Chinese expert consensus on the combined use of antiviral drugs for influenza, authoritative guidelines currently recommend early, single-agent antiviral therapy as the standard regimen for most patients. However, the consensus acknowledges that anti-influenza virus monotherapy has problems such as drug resistance and poor therapeutic effect in certain situations.

Combination therapy is being explored to address these challenges, though the consensus emphasizes that decisions about combined antiviral therapy should be made for specific populations and under expert guidance. This remains an active area of clinical research, particularly for severely ill or immunocompromised patients.

Supportive Care

Most otherwise healthy individuals with uncomplicated influenza recover with supportive care alone:

1. Rest to conserve energy for immune response
2. Hydration with water, clear broths, or oral rehydration solutions to prevent dehydration
3. Over-the-counter medications for symptom relief:

1. Acetaminophen or ibuprofen for fever and myalgias
2. Cough suppressants (dextromethorphan) for bothersome cough

• Expectorants (guaifenesin) to loosen mucus

1. Avoid aspirin in children and adolescents due to risk of Reye’s syndrome

When Hospitalization Is Required

Hospitalization may be necessary for patients with:

1. Severe dehydration (unable to maintain oral intake)
2. Hypoxia (low oxygen saturation)
3. Respiratory distress
4. Altered mental status
5. Significant exacerbation of underlying chronic conditions
6. Complications such as pneumonia or secondary bacterial infection

In hospitalized patients, treatment includes intravenous hydration, supplemental oxygen if needed, and antiviral therapy (oseltamivir is the preferred agent for hospitalized patients, regardless of time since symptom onset).

Complications of Influenza

Influenza is not a benign illness. Complications can be severe, particularly in high-risk populations.

Pulmonary Complications

• Primary influenza viral pneumonia: The most serious complication. Onset is typically 2-5 days after symptom onset, with rapid progression to respiratory failure. A 2017 study found that 76.4% of hospitalized patients with laboratory-confirmed influenza had detectable viruses, and among these, influenza A was found in 42.1% and influenza B in 18.6% .
• Secondary bacterial pneumonia: Occurs 5-14 days after initial improvement. Common pathogens include Streptococcus pneumoniae, Staphylococcus aureus (including MRSA), and Haemophilus influenzae.
• Acute respiratory distress syndrome (ARDS) : Severe lung injury requiring mechanical ventilation.
• Exacerbation of chronic lung disease (COPD, asthma, cystic fibrosis).

Cardiovascular Complications

• Myocarditis (inflammation of heart muscle)
• Pericarditis (inflammation of the sac surrounding the heart)
• Acute myocardial infarction (heart attack) – risk increases during acute influenza infection
• Worsening of pre-existing heart failure

Neurologic Complications

• Encephalitis (inflammation of brain tissue)
• Encephalopathy (altered brain function)
• Seizures, particularly in children
• Guillain-Barré syndrome (rare, occurring weeks after infection)

Other Complications

• Myositis and rhabdomyolysis (muscle breakdown)
• Acute kidney injury
• Multi-organ failure in severe cases
• Death

Prevention of Influenza

Vaccination: The Cornerstone of Prevention

The annual administration of the influenza vaccine is the most effective method for preventing influenza. Vaccination reduces the risk of influenza illness, hospitalization, and death.

Types of Influenza Vaccines

1. Inactivated Influenza Vaccine (IIV):The most common type, administered by intramuscular injection. Contains killed virus and cannot cause influenza. Approved for ages 6 months and older.
2. Recombinant Influenza Vaccine (RIV):Produced using recombinant DNA technology; does not use eggs or live virus. Approved for ages 18 years and older.
3. Live Attenuated Influenza Vaccine (LAIV):Administered as a nasal spray; contains weakened live virus. Approved for non-pregnant individuals ages 2-49 years without certain underlying conditions.

Who Should Be Vaccinated?

The CDC recommends annual influenza vaccination for all individuals aged 6 months and older who do not have contraindications. Vaccination is particularly important for high-risk groups and their close contacts.

Other Prevention Strategies

Non-Pharmaceutical Interventions (NPIs):

1. Hand washing with soap and water or alcohol-based hand sanitizer
2. Respiratory etiquette (covering coughs and sneezes with elbow or tissue)
3. Wearing masks in crowded indoor settings during influenza season
4. Staying home when ill (self-isolation)
5. Improving indoor ventilation (opening windows, using HEPA filters)

These NPIs, implemented during the COVID-19 pandemic, were highly effective at reducing influenza transmission, with a stronger suppressive effect on Influenza A than Influenza B .

Chemoprophylaxis:
Antiviral medications (oseltamivir) can be used for post-exposure prophylaxis in certain situations:

1. Unvaccinated high-risk individuals exposed to confirmed influenza
2. Institutional outbreak control (e.g., nursing homes)
3. Severely immunocompromised patients

Prophylaxis should be initiated within 48 hours of exposure and continued for 7-10 days.

Lifestyle and Management Tips for Influenza Patients

During Active Illness

For Mild to Moderate Illness (Managing at home):

1. Isolate to prevent spread

• Stay home from work, school, and public gatherings
• Avoid close contact with household members when possible
• Wear a mask if you must be around others
• Isolate until at least 24 hours after fever resolves without antipyretics

1. Rest aggressively

• Sleep allows the immune system to mount an effective response
• Avoid physical exertion even if you feel “well enough”

1. Hydrate thoroughly

• Fever increases insensible fluid losses
• Drink water, herbal tea, clear broths, or oral rehydration solutions
• Monitor urine color: pale yellow indicates adequate hydration

1. Manage fever and discomfort

• Acetaminophen or ibuprofen as directed
• Avoid aspirin in children and adolescents

1. Monitor for warning signs

• Difficulty breathing, chest pain, confusion, dizziness
• Symptoms that improve then suddenly worsen
• Persistent high fever (>3-4 days)

1. Consider antiviral treatment

• Most effective when started within 48 hours of symptom onset
• Particularly important for high-risk individuals
• Contact healthcare provider promptly

When to Seek Medical Care

Seek medical evaluation if you have:

• Difficulty breathing or shortness of breath
• Chest pain or pressure
• Sudden dizziness or confusion
• Severe or persistent vomiting
• Symptoms that improve but then return with fever and worse cough
• High-risk status (age <2 or >65, pregnancy, chronic medical conditions) and flu symptoms

Recovery After Influenza

Following acute influenza infection, many patients experience persistent fatigue and cough for 1-3 weeks or longer. Resolution of these symptoms is gradual. Patients should:

• Gradually resume normal activities as tolerated
• Avoid premature return to exercise (risk of relapse)
• Watch for delayed complications (secondary pneumonia typically occurs 5-14 days after initial improvement)

Frequently Asked Questions (FAQs)

1. How long is the flu contagious?

Adults with influenza are typically contagious from 1 day before symptoms begin until approximately 5-7 days after symptom onset. Children and severely immunocompromised individuals may remain contagious for longer (10 days or more). The highest contagious period is during the first 3-4 days of illness, when fever is present. Individuals should stay home until at least 24 hours after fever resolves without the use of fever-reducing medications.

2. How effective is the flu vaccine?

Vaccine effectiveness varies by season depending on the match between vaccine strains and circulating viruses. In the 2023/24 season, the inactivated influenza vaccine demonstrated effectiveness of 51-54% against influenza A and 56-60% against influenza B in children . While not perfect, the vaccine remains the most effective method for preventing influenza . Even when the vaccine does not prevent infection entirely, it significantly reduces the severity of illness, risk of hospitalization, and risk of death. For older adults and those with chronic conditions, these benefits are particularly important.

3. What is the difference between flu and COVID-19?

Influenza and COVID-19 are caused by different viruses (influenza viruses vs. SARS-CoV-2) and have different incubation periods. Symptoms overlap significantly—both cause fever, cough, fatigue, myalgias, and respiratory symptoms—but COVID-19 is more likely to cause loss of taste or smell. Laboratory testing (RT-PCR or antigen tests) is required to distinguish between them definitively. Both can cause severe illness, particularly in high-risk populations. Co-infection with both viruses is possible and associated with worse outcomes.

4. When should I take antiviral medication for the flu?

Antiviral medications (oseltamivir, baloxavir) are most effective when started within 48 hours of symptom onset. They are recommended for:

• All hospitalized patients with influenza (regardless of time since onset)
• High-risk outpatients (elderly, young children, pregnant women, those with chronic diseases, obesity)
• Patients with severe or progressive illness
  Early antiviral treatment is “key to reducing the rate of severe illness and death” . For otherwise healthy individuals with mild, uncomplicated influenza, supportive care alone is usually sufficient, though antivirals can still shorten symptom duration if started early.

5. Can I get the flu from the flu vaccine?

No. The injected influenza vaccine contains inactivated (killed) virus, which cannot cause influenza. The nasal spray vaccine (LAIV) contains weakened live virus that is temperature-sensitive (replicates only in the cooler nasal passages, not in the lower airways) and does not cause influenza illness in healthy individuals. Mild side effects such as sore arm, low-grade fever, and muscle aches, can occur and represent the immune system responding to the vaccine, not influenza infection. These symptoms typically resolve within 1-2 days.