“I got the shot, but when will it start working?” “What happens if I catch the flu right after getting vaccinated?”
The influenza vaccine does not take effect immediately after administration. It requires a certain period for antibodies to be created in the body, enabling it to fight off the influenza virus. Knowing this “time until effectiveness” is a crucial point for protecting yourself in time for the peak of the epidemic.
Complete Summary: When Does the Influenza Vaccine Take Effect? (2025 Latest)
| Days After Vaccination | State of Effectiveness | Real-World Experience |
| Day of Shot – Day 3 | Virtually zero | Not effective at all yet. |
| Day 4 – Day 7 | Starts to work (around 20–40%) | Antibodies slightly begin to rise. |
| Day 8 – Day 10 | Rapid surge up to 50–70% | From here, the effect of “milder illness even if contracted” is felt. |
| 2 Weeks Post-Shot (Day 14) | Reaches Peak → Maximum Effect | The real “prevention season” starts here. |
| 3 Weeks – 1 Month | Peak is largely maintained | The strongest period. |
| 3 Months Post-Shot | Slowly declines (to about 70–80%) | Still highly effective. |
| 5 Months Post-Shot | Declines to about half | Weakens around March. |
The “Truly Protected Period”
| Date of Vaccination | Date Effect Reaches Peak | Period of Strong Protection (70%+ Effectiveness) |
| October 15th Shot | October 29th | Early November to Mid-March |
| November 1st Shot | November 15th | Late November to Early April |
| November 30th Shot | December 14th | Late December to Early May (Just in time) |
| December 20th Shot | January 3rd | Mid-January to May (Only catches half of the epidemic) |
Conclusion
- Effectiveness begins: Approximately 10 days after vaccination.
- Maximum effectiveness is reached: Exactly 2 weeks (14 days) after vaccination.
- You must get the shot by mid-December to be protected in time for the main part of the epidemic!
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) |
