The influenza vaccine is one of the safest vaccines in the world. Side effects are almost always mild: “Risk of NOT being vaccinated” > “Risk of side effects.”
1. Frequency and Types of Side Effects (Data from MHLW & PMDA) Side Effect Frequency (Occurrence Rate) Details Local Reaction (Injection Site) 30-50% Systemic Reaction 5-15% Severe Side Effects Extremely Rare (Less than 1 in a million)
2. Details of Local Reactions (Injection Site) Symptom Time of Onset Duration Management Pain Immediately after injection ~ 24 hours 1-2 days Cool the area and rest Redness/Swelling A few hours ~ 1 day after 2-3 days Cold compress Induration (Lump) 1-3 days after Approx. 1 week Disappears naturally Lymph Node Swelling (Armpit) Rare Several days No issue
“Arm too sore to lift” → Recovers in 1-2 days.
3. Details of Systemic Reactions Symptom Time of Onset Duration Management Fever (37.5-38.5°C) 6-12 hours after injection 1-2 days Calonal (Acetaminophen) Malaise/Fatigue Same day ~ next day 1-3 days Rest, hydration Muscle/Joint Pain Same day ~ next day 1-2 days Stretching, warmth Headache Same day ~ next day 1 day Pain reliever (consult a doctor) Specific to Children Crying/Irritability Several hours Cuddle, hydration
You might think, “Did I catch a cold?”, but it stops abruptly after 2 days.
4. Severe Side Effects (Extremely Rare) Side Effect Occurrence Rate Symptoms Action Anaphylaxis 1-2 in a million Within 15 minutes of injection: ⋅ Hives ⋅Difficulty breathing ⋅ Drop in blood pressure Immediate ⋅ Adrenaline injection at a medical facility Guillain-Barré Syndrome (GBS) 1 in a million 1-3 weeks after injection: ⋅ Numbness/tingling in limbs ⋅ Muscle weakness Hospitalization ⋅Immunoglobulin treatment Encephalitis/Meningitis Almost zero High fever + impaired consciousness Immediate hospitalization
Medical facilities require “15-30 minutes of waiting” → To manage anaphylaxis.
5. People “More Prone” to Side Effects Risk Factor Reason First-time vaccination Stronger immune response Children (especially under 5) Active immune system Allergy history Prone to mild reactions Previous year’s side effects Possibility of recurrence this year
6. Difference between Side Effects and “Real Flu” Item Vaccine Side Effect Real Influenza Fever 37-38°C range (1-2 days) 39°C or higher (3-5 days) Cough/Runny Nose Almost none Present Muscle Aches Mild Whole body shaking Infectivity Zero Contagious to others
“Catching the flu from the vaccine” is an urban legend!
7. Managing Side Effects (At Home) Symptom Management Pain/Swelling Cold compress (ice pack wrapped in a towel) Fever/Malaise Calonal (Acetaminophen) *Children’s dose based on weight Induration (Lump) Apply warmth (after the 3rd day) Anxiety Call a medical facility for consultation
“Fever lasting 3 days” or “Difficulty breathing” → Seek medical attention immediately!
8. Side Effect Occurrence Rate (By Age) Age Local Reaction Fever Severe Reaction 6 months ~ 5 years 40% 10-15% Extremely Rare 6 ~ 64 years 30-50% 5-10% Extremely Rare 65 years and older 20-30% 2-5% Extremely Rare
Older people have fewer side effects!
9. Past Misconceptions and Truths Misconception Truth “Vaccine gives you the flu” × It contains dead virus, so it cannot cause infection. “Can’t get vaccinated if you have egg allergy” × Egg component is less than 0.1μg (almost zero). “Side effects = danger” × 99.9% are mild.
10. Procedures at Medical Facilities Procedure Content Before injection Allergy/health check on medical questionnaire After injection 15-30 minute wait (for anaphylaxis observation) Consultation for side effects Free consultation hotline (PMDA: 0120-149-931)
Summary: “Iron Rules” of Side Effects
99% result in “pain + mild fever” and end in 2 days.
Severe reactions are less than 1 in a million.
15 minutes of waiting at a medical facility → Safe. Side Effects << Risk of Influenza. “Fear of side effects” is 100 times less scary than “hospitalization due to pneumonia!”
