Main text
Prior to 2021, Republic of Korea
had experienced typical seasonal epidemics of influenza, with some
seasons showing higher rates of influenza-like illness. Typically,
seasonal flu epidemics in the country are reported between December and
March [1]. In the 2019-2020 season, there was a relatively high
number of ILI rates. Notably, during the 2020-2021 and 2021-2022
seasons, the ILI rates had remained below the epidemic threshold, as
Figure 1 illustrates. There had been continuous concern for the possible
”twindemic” since the emergence of COVID-19. In the 2021-2022 season,
this concern had grown with the ease of non-pharmaceutical
interventions, including social distancing measures. ”Twindemic” is a
term used to describe the simultaneous occurrence of two different
epidemics: COVID-19 and the seasonal influenza. It is used to stress the
possible subsequent burden on healthcare systems and on the overall
society.
The 2022-2023 flu season in the Republic of Korea has shown a similar
pattern to seasons prior to the COVID-19 pandemic. Despite an increase
in the ILI baseline since week 40, no early increase was observed. ILI
rates increased above the seasonal threshold (4.9 ILI cases per 1,000
outpatients) in week 40, gradually increasing until week 49, and showing
a typical exponential increase starting from week 50, similar to the
pattern before the COVID-19 pandemic. The peak was observed in week 53
and has been decreasing since then. This reoccurrence of the typical flu
epidemic in the Republic of Korea is not surprising, given the lifting
of overall social distancing measures and reduced population immunity to
seasonal influenza due to the absence of a flu epidemic in the 2020-2021
and 2021-2022 seasons [2]. Influenza vaccination rates have been
stable during the COVID-19 pandemic [3]. Nevertheless, the indoor
mask mandate has been maintained, which may have prevented an early
surge of the seasonal flu epidemic, unlike in other countries, including
Australia and the United States [4, 5, 6].
The impact of the resurgence of the seasonal flu in the midst of the
Omicron variant-driven COVID-19 pandemic has not been as catastrophic as
feared in the country. There is a chance for a spring season second peak
since the indoor mask mandate is set to be lifted by the end of January
2023. Therefore, close monitoring of seasonal flu and COVID-19 trends,
as well as continuous encouragement of vaccination for both highly
transmissible pathogens, is needed. At present, the worst-case scenario
appears to have been averted. Nevertheless, it is imperative that we
continue to exercise caution, as it is currently too early to lower our
guard.