Seoul flu season starts about 3 weeks early; high-risk groups urged to vaccinate
In short
Seoul's influenza patients began rising in week 35, in late August, about three weeks earlier than last year and 11 weeks earlier than 2019, the Seoul Institute of Health and Environment said on Oct. 4. Virus was detected in sewage from week 31, in late July, six weeks earlier than last year, and the 2025–2026 season lasted 16 weeks against five weeks a year earlier. The Korea Disease Control and Prevention Agency issued an influenza epidemic advisory on Sept. 11 and free national vaccination began on Sept. 21 for children, pregnant women and older adults.
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Seoul's influenza season began about three weeks earlier than last year, with patient numbers increasing from week 35, in late August, the Seoul Institute of Health and Environment said on Oct. 4. Compared with 2019, before the COVID-19 outbreak, the increase came 11 weeks earlier, the institute said. It said influenza virus was detected in sewage from week 31, in late July, six weeks earlier than last year. The virus currently circulating is type A(H1N1), and detections are increasing in both patient and sewage samples, according to the institute. [ 1 , 2 , 4 , 5 ]
The institute, which tests samples from patients with respiratory symptoms through the year and analyses virus in sewage to track respiratory infectious disease trends in Seoul, said the season is also lasting longer. The 2025–2026 influenza season ran 16 weeks, more than three times the five weeks of the 2024–2025 season, it said. Asia Economy quoted the institute as saying the season is lasting longer and occurring across all age groups, so children, pregnant women and older adults need particular attention. [ 1 , 2 , 4 , 5 ]
The Korea Disease Control and Prevention Agency issued an influenza epidemic advisory on Sept. 11 in response to the early season, and national free vaccination began on Sept. 21. Yonhap reported that the program covers children aged 6 months to 14 years, pregnant women and people aged 65 and older, with a phased rollout: children needing two doses and pregnant women from Sept. 21, children needing one dose from Sept. 28, and older adults from Oct. 12 for those 75 and older, Oct. 15 for ages 70 to 74 and Oct. 19 for ages 65 to 69. Asia Economy reported that the eligible groups can receive one dose of a trivalent vaccine at designated medical institutions and public health centres starting Sept. 21, and the documents do not reconcile the two accounts. [ 1 , 2 , 4 , 5 ]
Park Joo-sung, director of the institute, said it would work with the KDCA to monitor respiratory infectious diseases closely and asked residents to be vaccinated and to follow hand washing, mask wearing and other personal prevention measures. The institute also recommended cough etiquette and said people with fever and respiratory symptoms should visit a medical institution. [ 2 , 4 , 5 ]
Newsis published charts provided by the institute on its 2018–2026 influenza pathogen survey and on sewage monitoring results for weeks 34 to 38. Korea Economic Daily's headline likewise reported the season starting three weeks early and lasting three times as long, while its article body carried unrelated Seoul city stories. [ 3 , 6 ]
Why it matters
An earlier and longer flu season means Seoul's clinics and hospitals can expect respiratory cases over a longer stretch of the autumn, and the institute names children, pregnant women and older adults as the groups needing particular attention. The main response described in the documents is the national free vaccination program, supplemented by hand washing, cough etiquette and mask wearing. The findings come from the city institute's year-round patient and sewage testing, and the documents give no patient counts, so the practical impact described is largely local.
Key facts
- Seoul's influenza patients began increasing in week 35, in late August, about three weeks earlier than last year and 11 weeks earlier than 2019, the Seoul Institute of Health and Environment said. [ 1 , 2 , 4 , 5 ]
- Influenza virus was detected in sewage from week 31, in late July, six weeks earlier than last year. [ 1 , 2 , 4 , 5 ]
- The virus currently circulating is type A(H1N1), with detections rising in both patient and sewage samples. [ 1 , 2 , 4 , 5 ]
- The 2025–2026 influenza season ran 16 weeks, more than three times the five weeks of the 2024–2025 season. [ 1 , 2 , 4 , 5 ]
- The Korea Disease Control and Prevention Agency issued an influenza epidemic advisory on Sept. 11, and national free vaccination began on Sept. 21. [ 1 , 2 , 4 , 5 ]
- The free program targets children aged 6 months to 14 years, pregnant women and people aged 65 and older. [ 2 , 4 , 5 ]
- Park Joo-sung, director of the Seoul Institute of Health and Environment, asked residents to get vaccinated and to follow hand washing, mask wearing and other personal prevention measures. [ 2 , 4 , 5 ]
Confirmed by several sources
- Seoul's influenza patients began increasing in week 35, in late August, about three weeks earlier than last year and 11 weeks earlier than in 2019, according to the Seoul Institute of Health and Environment. [ 1 , 2 , 4 , 5 ]
- Influenza virus was detected in Seoul's sewage from week 31, in late July, six weeks earlier than last year. [ 1 , 2 , 4 , 5 ]
- The circulating virus is type A(H1N1), and detections are increasing in both patient and sewage samples. [ 1 , 2 , 4 , 5 ]
- The 2025–2026 influenza season lasted 16 weeks, more than three times the five weeks of the 2024–2025 season. [ 1 , 2 , 4 , 5 ]
- The Korea Disease Control and Prevention Agency issued an influenza epidemic advisory on Sept. 11 in response to the early season, and national free vaccination began on Sept. 21. [ 1 , 2 , 4 , 5 ]
- The institute said children, pregnant women and older adults are high-risk groups who need particular attention, and urged vaccination. [ 1 , 2 , 4 , 5 ]
- Park Joo-sung, director of the Seoul Institute of Health and Environment, said the institute would work with the KDCA to monitor respiratory infectious diseases and asked residents to follow prevention measures. [ 2 , 4 , 5 ]
Still unclear
- Whether Seoul's free vaccination program opened to all eligible groups on Sept. 21 or followed a phased schedule by group and age. Asia Economy says the program started Sept. 21 for children aged 6 months to 14 years, pregnant women and people 65 and older, who can receive one trivalent dose; Yonhap and Hankook Ilbo list separate start dates by group and age (Sept. 21, Sept. 28, Oct. 12, Oct. 15 and Oct. 19). The documents do not reconcile the two accounts.
- What caused the earlier start and longer duration of the influenza season. The documents report the timing and the year-on-year comparisons but give no cause or explanation.
- The number of influenza patients, hospitalisations or deaths in Seoul. The documents give week numbers and comparisons but no case counts or severity figures.
What local media are saying
Timeline, local time
- Dong-A Ilbo reports that Seoul's influenza season began about three weeks earlier than last year and that the city urged high-risk groups to be vaccinated. [ 1 ]
- Asia Economy, Newsis and Yonhap carry the Seoul Institute of Health and Environment's findings, including the week 35 rise in patients and the week 31 sewage detections. [ 2 , 3 , 4 ]
- Newsis updates its article. [ 3 ]
- Hankook Ilbo reports the findings, including the sewage detection in late July and the phased vaccination dates by age group. [ 5 ]
- Korea Economic Daily's headline reports the season starting three weeks early and lasting three times as long. [ 6 ]