In this study, relatively few excess deaths occurred during the analysis at the national level until December 2020. However, excess mortality was observed in 2021 and 2022, especially from July 2021. It is believed that the proportion of deaths caused by COVID-19 was relatively higher than in other years because effective strategies were not yet in place to cope with the new virus. This study only includes 2022 data from January through May, so it is hard to compare 2022 with previous years.
Although the COVID-19 case fatality rate (CFR) was not high (Supplemental Fig. 2), it was confirmed that the excess mortality increased over time relative to that of previous epidemic waves, as the number of confirmed cases increased. The number of deaths from causes other than COVID-19 may have increased for various reasons, such as the effect of vaccination, patients’ co-morbidities, patients not visiting the hospital because of lockdown or concerns about infection, or patients having difficulties presenting to hospitals because of hospitals’ COVID-19 policies. This suggests that there was a strong indirect impact of COVID-19 on overall mortality.
Many studies have investigated excess mortality at the national (10, 11) or regional (12) levels. According to a Polish study(13), the epidemic had a significant impact on the rise in mortality in 2020 and 2021, which 90% of the observed increases in mortality were attributable to COVID-19 direct victims. Additionally, mortality from cardiovascular diseases, neurological diseases, and mental problems showed the greatest increases in the group of fatalities not linked to COVID-19 in the same period (13). A study published in 2021 (14) analyzed excess deaths in Korea using national and regional data and found that the mortality trends in 2020 were similar to historical trends.
Other research has investigated deaths from various causes, including COVID-19, during the epidemic (15), and roughly 13% of the excess mortality observed in 2020 was due to non–COVID-19 causes in certain groups of people, such as males and young people (16).
The gap between excess deaths and reported deaths could be affected by numerous factors, such as medical accessibility, lockdown policies, and the number of COVID-19 tests. The mortality rates of patients with chronic diseases have declined over the epidemic period in Europe (17), likely in large part due to the most vulnerable individuals dying the earlies after the onset of the COVID-19 epidemic. In contrast, the number of suicides increased during the same period in Japan (18). The net effects associated with excess deaths are difficult to characterize.
One study compared CFRs between the United Kingdom, Taiwan, Italy, Spain, New Zealand, United States, Korea, and Japan and determined that the highest COVID-19 mortality rates were among people over 60 years old, particularly in the United States, United Kingdom, Spain, Italy, Japan, Korea, and New Zealand (in descending order) (19).
In Korea, the COVID-19 CFR and mortality rate is relatively low compared with other high-income countries; however, the COVID-19 mortality was higher in 2021 than in 2020 (20), which could have contributed to excess mortality.
There was also a study investigating excess deaths conducted in other countries that analyzed the results for each of the first and second epidemic waves. The mortality rate in second waves were low because of improvements in the preparedness of the healthcare services (21).
The results of a World Health Organization (WHO)(22) analyzed that there was no excess mortality in 2020 in Korea, however, excess mortality occurred in 2021 and the entire period of 2020–2021, which aligned with those of our study. It showed excess mortality occurred during 2020–2021 in the USA, UK, and Korea (140, 109, and 6 cases per 100,000 persons, respectively), whereas mortality decreased in Japan and New Zealand (which had excess mortality calculations of − 8 and − 28, respectively)(22).
A study published in The Lancet (23) determined that excess mortality occurred in Korea in 2020–2021; however, this result was not statistically significant. Their analysis showed that excess mortality occurred in the USA, UK, Japan, and Korea (again, the Korean finding was not statistically significant).
When comparing the results of a study conducted in ‘our mortality data’(5) with our study results (Supplemental Fig. 3), the P-score trends were similar between the analyses, and unlike other countries, the P-scores determined in our study were higher in the later COVID-19 epidemic period in Korea. Daily excess mortality data are shown in Supplemental Fig. 4, which also shows a similar P-score pattern. The later increases in excess mortality in Korea may be related to increased demand for ICU care and medical resources of both COVID-19 and non-COVID-19 critically ill patients.
There may have been fewer opportunities for ICU admissions of non-COVID-19 patients during the pandemic because of the ICU admission of seriously ill COVID-19 patients, which may have contributed to the decline in ICU admissions for non-COVID-19 patients. During the epidemic, a large proportion of medical resources, including ICU care at tertiary hospitals, have been allocated to caring for critically ill patients with COVID-19. The number of facilities with available ICU space and resources (such as mechanical ventilators, extracorporeal membrane oxygenation, and continuous renal replacement therapy) for non-COVID-19 patients might have also been reduced. This disparate distribution of medical resources would have acted as a substantial burden causing excess mortality and morbidity. Further detailed data about ICU resources would be needed.
Our study also showed increased mortality from with major diseases in 2020 and 2021, even if the number of admissions was decreased or maintained. This suggests that the quality of medical care was compromised during the epidemic period because of prolonged excessive working hours among healthcare workers, accumulated fatigue, and a scarcity of medical resources. Previous research revealed that in-hospital mortality was increasing as the numbers of nursing staff and intensivists decreased (24). We can estimate that this deterioration in the quality of medical care has contributed to excess mortality.
As far as we know, this was the first study analyzing the indirect effects of COVID-19 with such a long analysis period and that investigated the association between excess deaths and ICU admissions. However, there were some limitations. First, mortality is affected by many variables, such as population structure and aging, in addition to time and region, and we could not account or adjust for all of these factors. However, we have analyzed published mortality data from the previous five years as references and determined the annual trends and seasonal variation, which are the most influential factors. Second, we could not reflect the COVID-19 variants and vaccination status in our findings. The prevalent COVID-19 variants (alpha, beta, gamma, delta, and omicron) were different in each period and that the vaccination policies varied between periods, and these variations might have affected the excess mortality findings.
Omicron, for example, is known to be associated with mild clinical manifestation (25) and low mortality rates. KDCA announced that the incidence rate of the Omicron variant was about 11.18 times (95% CI: 8.90-14.04) higher than that of the Delta variant, however, the fatality rate of the Omicron variant was about 48% (adjusted relative risk = 0.52, 95% CI: 0.32–0.87) lower than that of the Delta variant(26). However, the excess mortality during the omicron period was higher than that of the previous period. Third, the causes of death (other than COVID-19) were not specifically analyzed, and it would be important to find out which diseases contributed most to excess mortality. Forth, we couldn’t explain the reasons of the decline in ICU admission. However, this study could serve as a basis for further analysis.
Excess mortality is a major issue of the public health in this new infectious disease era. Despite these limitations, this study analyzed excess mortality during the COVID-19 epidemic with long-term data, and determined the causes of excess mortality by identifying the number of ICU admission and the number of hospitalizations for major diseases with deaths. If medical resources are limited to existing patients due to the emergence of new infectious diseases, this would be a factor that can have a considerable impact on the increase in mortality.