Influenza B Spreads Early, KCDA Warns of Reinfection Risk

Even if you've just had the flu, you could catch another one — and it might be a different strain. Influenza B viruses are spreading earlier than usual, raising the risk of reinfection even for people who recently recovered from influenza A. “I’d just barely recovered after coming down with the flu a month ago, and now I have it again,” said a 39-year-old office worker surnamed Kim. Kim said their entire family suffered from influenza, starting with their middle school-aged daughter in early December, only for her to be diagnosed with the flu again last week. “I thought the flu was over and felt relieved, but I never expected to go through it twice in a month,” said Kim. “It’s hard to believe this can happen.” After an early-winter surge of influenza A hit a 10-year high before easing, influenza B is now spreading earlier than usual. Health authorities are warning that people who caught the flu late last year can still be infected again, depending on the virus type. Influenza infections, which had been declining since mid-November, started trending upward again in the second week of 2026, from Jan. 4 to 10, according to the Korea Disease Control and Prevention Agency (KDCA). During that period, the number of flu-like illness patients reported by primary care clinics rose to 40.9 per 1,000 outpatients, up from 36.4 the previous week — far above the epidemic threshold of 9.1. The rise has been driven largely by influenza B. Respiratory sample analysis showed the overall influenza virus detection rate in the second week slipped slightly from the previous week to 33.5 percent, but the breakdown by type shifted sharply. At the end of last year, influenza A accounted for 36.1 percent and influenza B for 0.5 percent. By the second week of this year, influenza A fell to 15.9 percent while influenza B climbed to 17.6 percent, overtaking type A. Influenza B typically peaks from late winter to early spring, but the KDCA said this season appears to be starting sooner than expected. “Influenza B is showing an earlier-than-usual pattern, raising concerns that flu infections — which had been declining — could increase again,” the agency said. By age group, cases were concentrated among children and teenagers. In the second week, the rate of suspected flu cases was highest among ages 7 to 12 at 127.2 per 1,000 outpatients, followed by 13 to 18 with 97.2 and 1 to 6 with 51.0. When outbreaks persist among school-age children, they often spread to households and workplaces with a time lag. Influenza is an acute respiratory infection that is broadly classified into influenza A and B. Symptoms are largely the same for both: after an incubation period of about one to four days, patients typically develop a fever of 38 degrees Celsius (100 degrees Fahrenheit) or higher, along with cough, sore throat, headache, muscle aches and runny nose. Children may also experience gastrointestinal symptoms such as vomiting or diarrhea. Fever usually lasts three to four days, though cough and sore throat may persist for several days longer. The concern is that influenza may not end with respiratory symptoms. Complications such as ear infections or bacterial pneumonia are more common among people 65 and older, children, pregnant women, those with chronic illnesses and immunocompromised patients. In rare cases, severe complications such as myocarditis or encephalitis can occur, and underlying heart or respiratory diseases can worsen. Cases that deteriorate into sepsis — sometimes leading to amputations of arms or legs — also occur every year. The U.S. Centers for Disease Control and Prevention estimated that, as of Friday, at least 18 million people had been infected with the flu this season, with 230,000 hospitalizations and 9,300 deaths. In Korea, the number of deaths directly attributed to influenza is about 200 a year. But the KDCA and academic researchers estimate that when excess deaths linked indirectly to the flu — including pneumonia complications and worsening of existing health conditions — are included, roughly 2,000 to 3,000 people die each year in connection with influenza. Health authorities stressed that even those who have already had influenza A can be infected again with influenza B. “Even if you were infected with influenza A early in this winter’s outbreak, you can be reinfected with influenza B,” said KDCA Commissioner Lim Seung-kwan. “High-risk groups — including seniors 65 and older, children and pregnant women — who have not yet been vaccinated should get a flu shot even now.” This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom. BY RHEE ESTHER [[email protected]]