The side effects that occur after an influenza vaccination, such as “sore arm,” “fever,” and “malaise,” are essentially “proof that your immune system has properly awakened” and is preparing to fight the virus.
The true nature of the side effect is the release of inflammatory substances called “cytokines” by immune cells (macrophages) that recognize the foreign material (vaccine antigen) in the body. If these cytokines cause local blood vessels to dilate, it leads to arm swelling; if they stimulate the brain’s thermoregulatory center, it leads to fever. Fever is an “immune strategy” designed to activate the immune system and is by no means a sign of a weak body.
Scientific Mechanism of Influenza Vaccine Side Effects Explained Simply
| Symptom | What is Happening Inside the Body? | Why is it More Common in Children? | Why is the Second Dose Milder? |
| Injection Site Pain, Redness, Swelling | ① Needle causes physical damage to skin/muscle. ② Macrophages recognizing the vaccine antigen as a “foreign substance” flood the area with Cytokines (IL-1, IL-6, TNF-α) → Blood vessel dilation + inflammatory cell accumulation → Redness and swelling. | Children have thinner skin + their immune system reacts more sensitively. | The immune system already has memory cells from the first dose, suppressing an overreaction. |
| Fever ≥38∘C / Malaise | Cytokines (especially IL-6 and Prostaglandin E2) stimulate the brain’s thermoregulatory center → The body’s temperature setpoint temporarily rises (a strategy to activate immunity). | Children have higher cytokine sensitivity (react 10–20 times more than adults). | The enemy is “already known,” so the cytokine release is drastically reduced (to 1/3 to 1/10). |
| Headache / Muscle Pain | Cytokines circulating throughout the body cause inflammation in muscles/joints → Increase in pain substance bradykinin. | Children have less muscle mass, yet the reaction is strong. | Same as above. |
| Anaphylaxis (Extremely Rare) | IgE reacts to trace residual proteins (like egg protein) in the vaccine → Mast cells release a burst of histamine. | Extremely rare in children with severe egg allergies. | Can sometimes cause sensitization, making the second dose potentially riskier (thus caution is needed). |
Internal Body Timeline (Post-Vaccination)
| Time | What is Happening Inside the Body? |
| 0–30 Minutes | Physical damage from the needle + Antigen recognition begins. |
| 2–6 Hours | Macrophages start the cytokine burst → Arm begins to throb. |
| 6–12 Hours | IL-6 reaches the brain → Fever begins to rise (Peak is 8–24 hours later). |
| 24–48 Hours | Cytokines begin to decrease → Quick recovery. |
| 1 Week Post-Shot | Memory B cells and T cells are fully formed → The ultimate shield for the winter season is ready. |
2025 Vaccine Type “Cytokine Volume” Ranking (= Side Effect Strength)
| Rank | Vaccine | Cytokine Volume (Relative Value) |
| 1st (Strongest) | Fluad (High-Dose + Adjuvant) | Approx. 3–4 times |
| 2nd | Recombinant Vaccine | Approx. 1.5–2 times |
| 3rd | Standard Domestic Vaccine | Standard |
| 4th (Weakest) | Intradermal Injection | 0.5–0.7 times |
Summary
Side effects = “Proof that the immune system has properly awakened.”
→ A burst of cytokines means “Enemy detected! All troops deploy!”—a sign that the immune army is mobilizing.
→ The second dose is quieter because “We already know this enemy, only a small force is needed.”
Therefore, “The child who has a stronger side effect is the child who is the most strongly protected this winter!”(Scientifically, numerous data show that children who experience a slight fever often have higher antibody titers than those with zero side effects.)
At-Home Testing Kit Ranking (Combined COVID-19 and Influenza)
Here is the English translation of the ranking table for at-home diagnostic kits that test for both COVID-19 and Influenza A/B:
| Rank | Product Name (Manufacturer) | Targets Detected | Sensitivity (Detection Rate) (Estimate: 12-48h Post-Onset) | Result Time | Approximate Price (Per Test) | Suitability for Pregnant Women / Children | Key Features & Availability |
| 1 | Panbio COVID-19/Flu A&B | COVID + Flu A/B | COVID: Approx. 90% | Flu: Approx. 85% | 15 mins | ¥1,980 – ¥2,480 | ◎ (Shallow Nasal Swab) |
| 2 | クイックナビ-Flu+COVID | Same (COVID + Flu A/B) | COVID: 88% | Flu: 80–85% | 15–20 mins | ¥1,780 – ¥2,200 | ◎ (Designed for children’s use) |
| 3 | KBMラインチェック nCoV/Flu | Same (COVID + Flu A/B) | COVID: 85% | Flu: Approx. 80% | 15 mins | ¥1,480 – ¥1,980 | ◎ |
| 4 | イージードック Flu＆COVID-19 | Same (COVID + Flu A/B) | COVID: 87% | Flu: 82% | 15 mins | ¥2,200 – ¥2,600 | ◎ (Many cases of use by pregnant women) |
