When getting the influenza vaccine, the most common concern is the “side effects,” particularly pain at the injection site and fever. Knowing the symptoms and duration of side effects beforehand can help you approach vaccination with peace of mind.
Based on actual 2025 data, nearly 80% of recipients experience “injection site pain,” but this usually subsides within 1 to 3 days. Systemic side effects (malaise, mild fever) occur in about 10% to 25% of people, but the duration is short (1 to 2 days) and is far milder than the high fever from actual influenza (39–40°C lasting 5 days).
Influenza Vaccine Side Effects: 2025 Latest Data Summary (MHLW + Clinical Practice)
| Symptom | Frequency (2024–2025 Season Actual Data) | Peak Time | Duration | Management |
| Injection Site Pain | 60–80% (Most common) | Immediately – Next day | 1–3 days | Apply a cold compress + move the arm |
| Injection Site Redness/Swelling | 30–50% | Next day – Day 2 | 2–5 days | Apply a cold compress |
| Malaise/Fatigue | 10–25% | Next day | 1–2 days | Rest |
| Mild Fever (37°C range) | 5–15% | 6–24 hours post-shot | 1–2 days | Monitor |
| Fever ≥38∘C | 1–3% | Next day | Only 1 day | Tylenol/Paracetamol is OK |
| Headache/Joint Pain | 5–10% | Next day | 1–2 days | Tylenol/Paracetamol |
| Anaphylaxis (Severe Allergy) | 1–2 per 1 million people | Within 30 minutes post-shot | Immediate | Immediate response at the clinic |
Vaccine Type Side Effect Strength Ranking (2025)
| Rank | Vaccine | Estimated Side Effect Strength |
| 1st (Strongest) | Fluad (High-Dose for Elderly) | ★★★★☆ (Stronger injection site reaction) |
| 2nd | Recombinant Vaccine (New) | ★★★☆☆ (Slightly stronger) |
| 3rd | Standard Domestic 4-Valent (Subcutaneous) | ★★☆☆☆ (Normal) |
| 4th (Weakest) | Intradermal Injection (Fine Needle) | ★☆☆☆☆ (Hardly any pain) |
Side Effect Reality for Children (Under 13)
- The second dose tends to result in fewer reactions than the first dose.
- Approximately 5% to 8% of children develop a fever of ≥38∘C (lasting only 1 day).
- Crying is mostly due to the pain of the injection itself, and 90% of children are energetic the next day.
Symptoms That “Absolutely Require a Hospital Visit” (2025)
- Hives, difficulty breathing, or facial swelling within 30 minutes of vaccination → Immediately return to the clinic.
- A fever of ≥39∘C persists for 2–3 days after the shot.
- Joint or muscle pain so severe that you cannot walk persists for more than 3 days.
The Truth About Side Effects in One Sentence (2025)
“8 to 9 out of 10 people only experience ‘a sore arm + slight fatigue’.”
“Compared to the high fever of actual influenza (39–40°C keeping you in bed for 5 days), the side effects are less than 1/100th as severe.”
→ If you avoid the shot out of fear of side effects, you risk suffering 100 times more severely from the actual illness!
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) |
