A COVID-19 testing station in Seoul is crowded with people who seek to undergo virus tests on Thursday. (Yonhap)

Health authorities on Friday issued an advisory over a possible seasonal influenza outbreak across the country, the first of its kind since 2019, amid growing concerns of a possible "twindemic" -- a simultaneous outbreak of COVID-19 and the flu -- this fall and winter.

The Korea Disease Control and Prevention Agency (KDCA) said the advisory was issued after the national influenza-like illness (ILI) case rate was tallied at 5.1 per 1,000 people from Sept. 4 to last Saturday.

The advisory, the country's first since 2019, was issued earlier when compared with past ones announced around November or December.

South Korea has not seen a flu outbreak in the past two years due largely to the government-mandated social distancing schemes. The KDCA said it has strengthened its flu advisory standard due to concerns over the potential twindemic situation.

The agency requested child care facilities, schools and nursing facilities strengthen influenza prevention and management schemes.

The KDCA plans to begin administering seasonal flu shots to high-risk groups -- children aged 6 months through 13 years old, pregnant mothers and seniors over 65 -- next Wednesday at some 20,000 hospitals and medical facilities nationwide. (Yonhap)