When we think of influenza prevention, we usually imagine wearing a mask or staying home after a fever starts. However, the period of highest infectivity is actually different from what most people imagine.
Surprisingly, the peak time for influenza virus transmission to others is the day before the fever starts, when the person feels absolutely no symptoms. This means that a decision to “rest because I have a fever” is often made too late, and you have likely already spread the virus to those around you.
Influenza Infectivity: “When Does It Start? When Does It End?” Super Simple Table
| Timing | Strength of Infectivity | Feeling Level |
| Approx. 24 hours before onset | ★★★★★ (Strongest Peak) | You are spreading the most virus while still completely asymptomatic. |
| Day of Onset | ★★★★★ (Peak Continues) | The period immediately after the fever starts is the most dangerous. |
| 1–2 days post-onset | ★★★★ (Still very strong) | Caution needed for people who are still mobile despite the fever. |
| 3 days post-onset | ★★★ (Significantly reduced) | Infectivity weakens rapidly as the fever begins to drop. |
| After Fever Resolution | ★ to ★★ (Almost zero) | Adults are mostly non-contagious 1 day after the fever breaks. |
| What about children? | Weak infectivity remains for 3–7 days after fever resolution. | Longer caution period needed for elementary school age and younger. |
At-a-Glance Infectivity Calendar (Example)
| Monday | Tuesday | Wednesday | Thursday | Friday |
| Infected → | Incubation Period (Asymptomatic) | Fever of 38.5∘C starts at night (Onset) | Infectivity MAX | Still strong |
| Least likely to be noticed | Tuesday is the day you spread the most virus (to family/colleagues) | Most likely to seek rest |
✅ Summary
- The time you are most contagious is “the day before the fever starts.” ← Extremely Important!
- You remain a “super high-risk person” for the first 48 hours after the fever starts.
- Infectivity is almost zero 1 day (adults) / 2–3 days (children) after the fever has completely resolved.
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) |
