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You walk into the clinic for your flu shot and hear, “We only have the three-strain vaccine this year.”
Fewer strains must mean less protection, right?
Not quite. The strain that was dropped hasn’t been detected in years.
Here’s how the trivalent vs quadrivalent flu vaccine choice actually works.
The trivalent vs quadrivalent flu vaccine difference comes down to one component: B/Yamagata. Korea’s 2026-2027 National Immunization Program (NIP, the government-funded vaccination program) uses trivalent vaccines with WHO-recommended strains. For adults 65 and older, the US CDC prefers high-dose, adjuvanted or recombinant vaccines, but Korea’s NIP covers the standard dose only. Talk with your doctor about which fits you.
What’s the difference between a trivalent vs quadrivalent flu vaccine?
Short answer: trivalent shots target three viruses, quadrivalent shots target four.
The only difference is the B/Yamagata lineage.
A trivalent flu vaccine is a vaccine built to target two influenza A viruses (H1N1 and H3N2) and one influenza B virus (B/Victoria lineage).
A quadrivalent vaccine adds a fourth component, the B/Yamagata lineage.
| Component | Trivalent | Quadrivalent |
|---|---|---|
| Influenza A H1N1 | Included | Included |
| Influenza A H3N2 | Included | Included |
| Influenza B/Victoria | Included | Included |
| Influenza B/Yamagata | Not included | Included |
Source: World Health Organization (WHO), vaccine composition announcement for the 2026 southern hemisphere season, September 26, 2025
Is the trivalent flu vaccine less effective than the quadrivalent?
Under current flu conditions, the Korea Disease Control and Prevention Agency (KDCA) says there’s no difference in protection between the two.
The reason is simple: B/Yamagata has effectively disappeared.
WHO says this lineage hasn’t been detected since March 2020.
It also says including a B/Yamagata antigen is no longer warranted.
It no longer issues updated recommendations for that component.
Three reasons health agencies are moving to trivalent vaccines:
- No B/Yamagata detections have been reported since March 2020.
- WHO no longer sees a need for a B/Yamagata antigen.
- Korea’s 2026-2027 NIP and all US flu vaccines this season are trivalent.
Source: WHO (September 2025), KDCA press release (August 2026), US CDC
Think of the trivalent shot as the same vaccine minus a component that no longer circulates, not a weaker quadrivalent. Real-world protection still varies each year with how well the vaccine matches circulating viruses and with your own immune system.
Should adults 65 and older get the high-dose flu vaccine?
The US CDC prefers one of three options for adults 65 and older: high-dose, adjuvanted or recombinant vaccine.
Korea’s NIP, however, supplies the standard-dose trivalent vaccine.
A high-dose flu vaccine is a vaccine for people 65 and older that contains four times the antigen of a standard dose to boost the immune response.
In Korea, Efluelda received approval from the Ministry of Food and Drug Safety (MFDS) in November 2023. Adults 65 and older get one 0.7 mL dose each year.
| Item | NIP trivalent | High-dose (Efluelda) |
|---|---|---|
| Dose type | Standard dose | 4x antigen |
| Who | 65+ and other groups | 65+ |
| Cost | Government-funded (free) | Paid out of pocket |
| Frequency | Once a year | Once a year |
The evidence is mixed.
In the Danish DANFLU-2 trial of adults 65 and older (2022-2025 seasons), the primary endpoint was hospitalization for influenza or pneumonia. It occurred in 0.68% of the high-dose group and 0.73% of the standard-dose group.
Relative vaccine effectiveness was 5.9% (95.2% CI -2.1 to 13.4), not statistically significant.
Looking at influenza hospitalization alone, the rates were 0.06% vs 0.11% (a secondary endpoint, relative effectiveness 43.6%).
The trial used a quadrivalent high-dose vaccine.
Source: US CDC, New England Journal of Medicine (NEJM), December 2025, Sanofi Korea press release, December 2023
You can’t assume the high-dose vaccine is better for everyone. If you’ve had a severe allergic reaction after a vaccine or you have a chronic condition, talk with your doctor or pharmacist before getting vaccinated.
Who can get a flu shot in Korea this season, and when?
Korea’s 2026-2027 NIP runs from September 21, 2026 to April 30, 2027.
For adults 65 and older, it starts October 12 with those 75 and over, then moves down by age group.
The National Immunization Program (NIP) is a government-funded vaccination program for priority groups.
It covers children from 6 months to 14 years, pregnant women and adults 65 and older.
You can get vaccinated at about 23,000 designated medical institutions or public health centers nationwide, regardless of where you live.
✅ Check the start date for your age group before you go
✅ Tell the clinic about any fever or allergy history
✅ Call ahead if you want a paid option such as a high-dose vaccine
For more on medications and supplements, see our Medications & Supplements category. (Internal link: replace after related posts are published)
Source: Korea Disease Control and Prevention Agency (KDCA), influenza NIP press release, August 2026
Frequently Asked Questions
1. Does the trivalent shot leave me unprotected against type B flu?
No. It includes B/Victoria. B/Yamagata hasn’t been detected since March 2020. Still, no vaccine blocks infection completely. It may help lower your risk of severe illness.
2. I’m over 65. Should I pick the free vaccine or the high-dose one?
The free NIP vaccine is standard-dose trivalent. The high-dose option is paid. Your health and budget both matter, so talk with your doctor.
3. Is the high-dose vaccine clearly better?
In DANFLU-2, the primary endpoint showed no significant difference. The secondary analysis of influenza hospitalization alone reported lower rates. Because the results are mixed, ask your doctor what fits you.
4. How long can I get the NIP flu shot this season?
Through April 30, 2027. Start dates differ by age group, so check yours first.
References
- Korea Disease Control and Prevention Agency (KDCA), “Influenza NIP this winter expanded to age 14” (올겨울 인플루엔자 국가예방접종, 14세까지 확대), 2026 Link
- World Health Organization, “Recommendations announced for influenza vaccine composition for the 2026 southern hemisphere influenza season”, 2025 Link
- US CDC, “Key Facts About Seasonal Flu Vaccine”, continuously updated page Link
- US CDC MMWR, “Prevention and Control of Seasonal Influenza with Vaccines: ACIP Recommendations, 2025-26 Influenza Season”, 2025 Link
- New England Journal of Medicine, “High-Dose Influenza Vaccine Effectiveness against Hospitalization in Older Adults” (DANFLU-2), 2025 Link
- Sanofi Korea, “Efluelda high-dose influenza vaccine for adults 65+ approved in Korea” (사노피 코리아, 에플루엘다 국내 허가), 2023 Link
This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Please consult a qualified healthcare provider if you have health concerns.
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