There were a total of 10 studies on respiratory viruses [6-15]. Among them, 9 studies were analyzed using data from the surveillance system operated by the Korea Disease Control and Prevention Agency (KDCA), and one study used data obtained from the National Health Insurance Service (NHIS) database. Most studies analyzed the epidemiology of respiratory viral infections during the pandemic compared to the 5 years prior to the COVID-19 pandemic. Four studies [8,10,13,14] analyzed all 8 viruses (adenovirus [ADV], parainfluenza virus, respiratory syncytial virus, influenza virus [IFV], human coronavirus, human rhinovirus [HRV], human bocavirus [HBoV], and human metapneumovirus) reported in the surveillance system data, while the other 6 studies [6,7,9,11,12,15] analyzed some of the viruses. Of those 6 studies, 4 were of IFV [6,9,11,12]. The results differed among studies due to differences in analytical methods and study periods, but all showed that respiratory virus infections after the start of the COVID-19 pandemic had decreasing trends. This is thought to be due to the effectiveness of NPIs used to control COVID-19 infections.

However, 2 studies reported that nonenveloped viruses (ADV, HRV, and HBoV) were continuously detected even after the introduction of NPIs. Why the effectiveness of NPIs differed between respiratory viruses remains unclear, but it may be due to the unique characteristics of the specific viruses, subtle differences in respiratory transmission modes and viral shedding durations, and differences in resistance against environmental conditions [8,14,20-22].

In Korea, the mandatory surveillance of group 1–3 infectious diseases is implemented according to communicable disease types. Two studies [7,15] compared the monthly incidence before versus after NPIs for infectious diseases with respiratory transmission monitored by the KDCA. Both studies included varicella, mumps, invasive pneumococcal disease, and pertussis. In one of the studies, scarlet fever was further analyzed [7]. The monthly incidence of notifiable respiratory diseases during the NPI period decreased remarkably compared to the pre-NPI period. The largest reductions were observed for mumps and chicken pox.

One study [6] compared hospitalization rates before and after NPI for COPD and asthma, which are chronic lung diseases, in addition to pneumonia and influenza. Hospitalization data were collected from the NHIS database. The study results reported that hospitalizations for COPD, asthma, and respiratory infections such as pneumonia and influenza decreased during the NPI era. In this study, the incidence of hospitalization for other acute and chronic diseases, such as diabetic ketoacidosis or hyperosmolar hyperglycemic state, intracranial hemorrhage, myocardial infarction, and cancer, did not decrease after NPI, which demonstrated that these findings were not attributable to changes in healthcare utilization caused by the COVID-19 pandemic.

Two studies [16,17] analyzed gastrointestinal infections using open data provided by the KDCA. Common analytical subjects of the 2 studies included norovirus and rotavirus for gastrointestinal viruses and Campylobacter spp., Clostridium perfringens, and nontyphoidal Salmonella for gastrointestinal bacteria. In one study, 3 enteric viruses (sapovirus, astrovirus, and enteric ADV) and 2 enteric bacteria (Staphylococcus aureus and enteropathogenic Escherichia coli) were further analyzed [16].

In both studies, the incidence of gastrointestinal viral infection significantly decreased after the introduction of NPI, whereas the incidence of gastrointestinal bacterial infections was not significantly reduced. This is thought to be because, in the case of the former, transmission occurs mainly by direct contact infection, whereas in the case of the latter, transmission is food-borne.

Kawasaki disease (KD) is an acute febrile vasculitis of childhood with the complication of coronary artery lesions [23]. The etiology of KD remains unknown, although its symptoms are thought to be related to an excessive immune system reaction triggered by an infectious agent in patients with genetic susceptibility [24]. Many observational epidemiologic studies have provided evidence of a link between KD and infection. They include clinical similarity to infectious disease in children, geographical and epidemic temporal clusters, seasonal trends, a strong relationship between KD and infection surveillance, and unusual recurrence [25]. However, no single pathogen has been identified to date.

Two studies [18,19] analyzed the incidence of KD before and after NPI in Korea using the NHIS claims database. One study [19] analyzed the overall KD incidence rate, while another [18] analyzed the KD-related hospitalization rate. The incidence of KD and the KD-related hospitalization rate decreased significantly after the introduction of NPIs. A decrease in KD after NPI has also been reported in the United States, Japan, and Singapore [26]. These findings emphasize the impact of extrinsic triggers such as common pediatric infections related to NPIs on the occurrence of KD.

Two studies [7,15] analyzed notifiable nonrespiratory infections. Both studies included hepatitis A. Hepatitis B was further analyzed in one study [15], while hepatitis C and carbapenem-resistant Enterobacterales were further investigated in another study [7].

Although the surge in hepatitis A observed in 2019 resulted in a relatively subtle decrease in hepatitis A during the NPI period, the observed cumulative incidence of hepatitis A was within the 95% confidence interval (CI) of the predicted value. The observed incidence of hepatitis C and carbapenem-resistant Enterobacterales during the NPI period remained within the 95% CIs of model predictions. There was also no change in the incidence of hepatitis B during the NPI period versus the pre-NPI period (2015–2019).