When the flu is spreading at work or school, many people worry, “Am I in the incubation period?” However, that worry may be entirely misplaced. The influenza incubation period is “completely asymptomatic” to a degree you might not imagine. The moment you feel symptoms like a slight fever, fatigue, or a scratchy throat, the incubation period is actually over, and the highly contagious “onset” of the disease has begun.
This article thoroughly explains the critical difference between the incubation period and the onset of illness, including the actual prevalence of symptoms and tips for telling them apart. Understand the correct action you should take and protect yourself from the trap of the “zero-symptom period,” which is the greatest source of infection.
Influenza Incubation Period Symptoms “Complete Summary Table”
| Symptom Item | Appears During Incubation? | Actual Prevalence (Perception) | Comment (Tip for Telling the Difference) |
| Fever/Slight Fever | Almost never | 99.9% Normal temperature | If your temperature is 37.0∘C or higher, you’ve likely already begun the onset. |
| Fatigue/Sluggishness | Almost never | 99% Zero | If you feel “sluggish,” you are likely just a few hours from the full onset. |
| Headache | Almost never | 99% Zero | If a headache starts, the illness has almost certainly begun. |
| Chills | Never | 100% Zero | Chills = the first step of the full onset. |
| Cough | Almost never | 99.5% Zero | If you have a cough, it’s either another cold or the illness has begun. |
| Runny/Stuffy Nose | Almost never | 99% Zero | If you have a runny nose, it’s 99% a common cold. |
| Sore Throat | Very rarely | About 1–3% | If it’s just a “slight weird feeling in the throat,” you might still be in the safe zone. |
| Muscle/Joint Pain | Almost never | 99.9% Zero | If this appears, the onset is 100% underway. |
| Appetite | Normal | You can eat normally | Sudden loss of appetite = Onset. |
The Real State During the Incubation Period (99% of People Experience This)
- Completely normal person.
- Work, school, sports, social events → Everything is done as usual.
- “I was perfectly fine yesterday…” is the standard phrase.
- Symptoms are zero, yet you are the most contagious (the biggest trap).
✅ Summary
Influenza’s Incubation Period = Almost 100% Completely Asymptomatic.
The moment you feel any symptom → The incubation period is over, and the onset has begun! Therefore, it is normal to “not realize it before the fever starts.”
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) |
