Incidence of Out-of-Hospital Cardiac Arrest on a Postholiday Weekday.

Importance: Although increased cardiovascular risk during holidays has been documented, whether this elevated risk persists into the postholiday transition period remains unclear. Objective: To quantify the incidence of out-of-hospital cardiac arrest on a postholiday weekday in South Korea. Design, Setting, and Participants: This nationwide cohort study analyzed data from the Out-of-Hospital Cardiac Arrest Surveillance database between January 1, 2013, and December 31, 2023. Adult out-of-hospital cardiac arrest cases occurring on weekdays were included, excluding those on holidays. Exposure: A postholiday weekday, defined as the first working day following any weekend or holiday period, compared with baseline weekdays (all other weekdays). Main Outcomes and Measures: The primary outcome was the out-of-hospital cardiac arrest incidence rate ratio (IRR) comparing a postholiday weekday with baseline weekdays, estimated using negative binomial regression. Subgroup analyses examined demographic and clinical factors, and additional analyses assessed dose-response associations according to the duration and type of preceding holidays. Results: Among the 203 471 adult participants (median [IQR] age, 71 [56-81] years; 130 348 [64.1%] male), 49 199 cases occurred on a postholiday weekday and 154 272 occurred on baseline weekdays. The daily out-of-hospital cardiac arrest incidence was significantly higher on a postholiday weekday than on baseline weekdays (median [IQR], 88 [78-98] vs 80 [71-89] cases), representing a 9% higher incidence (IRR, 1.09; 95% CI, 1.08-1.11; P < .001). Subgroup analysis revealed increased vulnerability among adults older than 65 years (IRR, 1.03; 95% CI, 1.02-1.04; P < .001), individuals with cardiac cause (IRR, 1.02; 95% CI, 1.01-1.04; P < .001), and those presenting with nonshockable rhythms (IRR, 1.03; 95% CI, 1.01-1.04; P < .001). A clear dose-response association was observed. Significant associations were observed after 2-day holidays (IRR, 1.10; 95% CI, 1.08-1.11; P < .001), 3-day holidays (IRR, 1.09; 95% CI, 1.03-1.15; P = .003), and holidays of 4 days or more (IRR, 1.10; 95% CI, 1.03-1.18; P = .008) but not after single-day holidays (IRR, 1.03; 95% CI, 0.98-1.08; P = .25). Weekend (IRR, 1.09; 95% CI, 1.07-1.11; P < .001) and mixed (IRR, 1.10; 95% CI, 1.06-1.14; P < .001) holidays were associated with a higher out-of-hospital cardiac arrest incidence, whereas public (IRR, 1.03; 95% CI, 0.97-1.09; P = .24) and temporary (IRR, 1.01; 95% CI, 0.89-1.13; P = .86) holidays were not. Conclusions and Relevance: In this nationwide cohort study, a postholiday weekday was associated with a significantly elevated out-of-hospital cardiac arrest incidence in South Korea, particularly after consecutive rest days and among vulnerable populations. These findings support enhanced emergency medical services preparedness, targeted public health messaging during holiday periods, and evaluation of preventive interventions for high-risk populations during postholiday transitions.