Influenza (Flu)

Influenza is a common disease that spreads every winter. While it's often dismissed as "just a cold," it's an infectious disease accompanied by sudden high fever and severe fatigue, with a risk of severe complications like pneumonia or encephalitis in individuals with pre-existing conditions or the elderly.

In this column, we will provide a clear explanation from a doctor's perspective, covering everything from influenza symptoms, differences from the common cold, testing and treatment, to prevention methods.

Key points of this article:

Influenza is characterized by "sudden high fever of 38-40℃ and severe malaise," and antiviral drugs are most effective when started within 48 hours of symptom onset. Annual vaccination in autumn is key to preventing severe cases.

What is Influenza?

Influenza is an acute respiratory infection caused by the influenza virus. In Japan, it typically spreads from November to March, with millions of patients affected in some years.

Differences from the Common Cold

Although often confused with the "common cold," influenza differs clearly in both the causative virus and the way symptoms appear.

Item	Common Cold	Influenza
Onset	Gradual	Sudden (rapid within hours)
Fever	Low-grade fever to around 38℃	High fever of 38-40℃
Systemic symptoms	Mild	Severe fatigue, muscle aches, joint pain
Nasal/Throat symptoms	Primary symptoms	Milder than systemic symptoms
Treatment drugs	Primarily symptomatic treatment	Antiviral drugs are effective
Peak season	Year-round	Mainly winter (Nov-Mar)

Types of Influenza (A, B, C)

Influenza that infects humans mainly comes in three types: A, B, and C. Types A and B are the most common.

• Type A: Most likely to cause outbreaks and mutates frequently. Tends to have a higher risk of severe illness.
• Type B: Generally milder than type A, but gastrointestinal symptoms are more common.
• Type C: Symptoms are mild and barely distinguishable from a common cold.

Main Symptoms of Influenza

Typical symptoms of influenza include a sudden high fever and general malaise. If the following symptoms appear suddenly, influenza should be strongly suspected:

• Sudden high fever of 38-40℃
• Severe general fatigue, muscle pain, joint pain
• Headache, chills, loss of appetite
• Sore throat, cough, runny nose
• Nausea, vomiting, diarrhea (more common with Type B)

Risk of febrile seizures and encephalopathy in children

Children may experience febrile seizures and, rarely, influenza encephalopathy. If symptoms such as disorientation, seizures, or persistent incoherent speech occur, seek emergency medical attention without hesitation.

Influenza Testing

Rapid Antigen Test

This test involves taking a swab of nasal or throat mucus to identify Type A and Type B. Results are available in about 15 minutes.

Best Timing for Testing

The accuracy of influenza testing varies depending on the time since symptom onset.

Time since onset	Test accuracy
~6 hours	Lower viral load, higher chance of false negatives
12~24 hours	Highest accuracy window
24~48 hours	Testable, antiviral drugs also effective
After 48 hours	Testable, but effectiveness of antiviral drugs decreases

Tip

Visiting too soon after symptoms appear carries a risk of false negatives. If symptoms are severe, it's recommended to visit half a day to a day after onset, or a retest may be necessary the next day even if you visit on the same day.

Co-testing with COVID-19

During periods of concurrent COVID-19 and influenza outbreaks, "COVID-flu co-testing" can be done to check for both with a single test. Early differentiation is important due to similar symptoms.

Influenza Treatment

Antiviral Drugs

Most effective when taken within 48 hours of symptom onset. The main four types of drugs are:

Drug Name	Form	Characteristics
Tamiflu	Capsule, dry syrup	Twice a day for 5 days. Can be used by children and pregnant women.
Inavir	Inhalation	Single inhalation completes treatment.
Relenza	Inhalation	Inhale twice a day for 5 days. Caution for asthma patients.
Xofluza	Tablet	Single dose completes treatment. Risk of resistant strain development.

Symptomatic Treatment

For fever, muscle pain, and headache, antipyretic analgesics (such as acetaminophen) are prescribed. Aspirin (Bufferin) is not used for children under 15 years old (due to the risk of Reye's syndrome).

Tips for Home Care

• Adequate hydration (water, oral rehydration solutions, sports drinks)
• Rest and sufficient sleep
• Maintain room temperature at 20-22℃ and humidity at 50-60%
• Easily digestible meals (porridge, udon, soup)

Influenza Prevention

Vaccination

The best way to prevent influenza is vaccination. The effects appear approximately two weeks after vaccination and last for about five months.

• Timing: Annually between October and November (before the peak of the outbreak)
• Effectiveness: In addition to preventing infection, it significantly reduces the risk of severe illness, hospitalization, and death.
• Target Audience: All individuals aged 6 months and older (especially the elderly, those with underlying conditions, pregnant women, and children).

Daily Infection Control

• Frequent hand washing and hand sanitizing
• Cough etiquette (wearing a mask)
• Avoiding crowds
• Proper humidity control (viruses become more active in dry conditions)
• Maintaining immunity through sufficient sleep and nutrition

Guidelines for Medical Consultation

Symptoms requiring immediate medical attention

• Sudden fever of 38℃ or higher
• Severe general fatigue, joint pain, muscle pain
• Fever when influenza is prevalent in your community
• High risk of severe illness due to pre-existing conditions

Symptoms requiring emergency medical attention

School/Work Attendance Restriction Period

Under the School Health and Safety Act, individuals diagnosed with influenza are subject to the following attendance restrictions:

Category	Period of Attendance Restriction
Preschool children	Until 5 days have passed since onset AND 3 days have passed since fever subsided
Elementary, junior high, and high school students	Until 5 days have passed since onset AND 2 days have passed since fever subsided
Adults (workplace)	No legal regulations, but staying home for at least 2 days after fever subsides is recommended

Frequently Asked Questions

Do I need to get an influenza vaccine every year?

Yes. The circulating virus strains change annually, so the vaccine composition is adjusted each year, and its effectiveness lasts for a limited period of approximately 5 months. Therefore, annual vaccination is recommended.

Can I still get influenza even if I'm vaccinated?

Yes, it's possible. The vaccine's effectiveness in preventing infection is around 60-70%, so it's not a complete prevention. However, vaccination significantly reduces the risk of severe illness.

If I'm diagnosed with influenza, should my family also get tested?

If family members who had close contact exhibit symptoms such as fever or malaise, we recommend they get tested. If they don't have symptoms, there's no urgent need for testing, but they should diligently practice infection control measures like handwashing and wearing masks.

Can I go out after my fever subsides?

Even after your fever subsides, the virus may still be present in your body, posing a risk of transmitting it to others. Please recuperate at home for the period specified by the School Health and Safety Act: "5 days after onset + 2 days after fever subsides (3 days for preschoolers)."

Can pregnant women get vaccinated?

Yes, vaccination is possible. Influenza during pregnancy can lead to severe complications, so vaccination is strongly recommended. Antiviral drugs (Tamiflu) can also be prescribed if necessary.

Summary

Influenza is an infectious disease characterized by "sudden high fever" and "severe systemic symptoms." Early testing and treatment can prevent severe complications, so if symptoms appear, do not hesitate to seek medical attention.

• Optimal timing for testing and treatment is within 12-48 hours of onset.
• Antiviral drugs are effective if started within 48 hours of onset.
• Annual vaccination in October-November is the cornerstone of prevention.
• For school and work, rest for "5 days after onset + 2 days after fever subsides."

Our clinic offers co-testing for influenza and COVID-19, antiviral prescriptions, and intravenous drips. If you are experiencing fever, please feel free to consult us.