Flu cases in South Korea rose nine folds in December from the previous month, most commonly appearing among children aged 7 to 12, government data showed Monday.

According to the weekly data of infectious diseases from the Korea Centers for Disease Control and Prevention, 71.8 patients among 1,000 outpatients had the flu in the 52nd week between Dec. 24 and Dec. 30, 9.3 times more than 7.7 patients in the 47th week between Nov. 20 and Nov. 26.

Influenza cases have continued to increase since the health authorities issued a flu watch on Dec. 1. The number went up from 19 in the 49th week to 30.7 in the 50th week, and to 53.6 in the 51st week last year. By age, influenza cases were most common among children aged 7 to 12, with 144.8 children per 1,000 outpatients infected with the flu.

The health authorities attributed the quick spread of flu to the outbreak of influenza A and influenza B at the same time.

The KCDC said it has detected the two types of influenza during the latest flu season that began from Sept. 3. In particular, detections of type A flu (H3N2) and type B surged. Among the 558 detected cases in total, 218 (39.1 percent) were type A (H3N2), 302 (54.1 percent) were type B, and 38 were type A (H1N1) pdm09.

“Usually, influenza A and B alternately appear – type A in the winter season, and type B in spring. But during the latest season, both types of influenza appeared at the same time. That has raised the chance of co-infections or alternate infections,” a KCDC official said.

Some critics said the World Health Organization (WHO) failed to predict what viruses will appear for the 2017-2018 season.

In every February and March, the WHO announces its predictions for the upcoming season to encourage vaccinations. Using the opposite seasons in the southern and northern hemispheres, the WHO selects viruses that had spread in the southern hemisphere and prepares vaccines against the chosen viruses for the northern hemisphere where Korea belongs.

In the March announcement for the 2017-2018 season in the northern hemisphere, the WHO recommended trivalent vaccines that include A(H1N1) pdm09, A(H3N2), and B(Brisbane/Victoria lineage).

However, the latest type B influenza was mostly Yamagata lineage, not Victoria lineage, according to KCDC.

The agency urged the public to get a flu vaccine shot as soon as possible, although belated. Type B influenza vaccine can make the body limitedly immune to different lineage types alternately, it noted.

“Flu cases are on the rise now, and the flu season may continue until late spring. It takes two to four weeks for the body’s immunity to fully work. So, we recommend getting vaccinated as soon as possible,” the KCDC official said.

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