**Background:** While young adults aged 18–24 account for a significant proportion of COVID-19 diagnoses, risk factors for hospitalisation and severe complications in this age group remain poorly understood. Prior studies have reported variable mortality rates ranging from 0.2% to 2.7% in hospitalised young adults, and the role of cardiovascular screening after COVID-19 in this population has been debated.
**Methods:** This retrospective cohort study combined data from two US registries: the American Heart Association (AHA) COVID-19 Cardiovascular Disease Registry (hospitalised patients from 152 hospitals, collected March 2020–April 2021) and the Outcomes Registry for Cardiac Conditions in Athletes (ORCCA) study (NCAA collegiate athletes from 42 colleges, collected September 2020–June 2021). The AHA cohort included 636 young adults (mean age 21.4 years, 59% female, 31% white, 27% Black, 32% Hispanic). The ORCCA cohort included 3653 athletes (mean age 19.9 years, 33% female, 65% white, 28% Black, 3% Hispanic). Asymptomatic patients (including those admitted for labour and delivery) were excluded from the AHA cohort. Primary outcomes were hospitalisation, death, MACE, and other severe clinical events. Univariable Firth logistic regression was used to identify predictors of death, MACE, and other severe events.
**Key Results:** Baseline characteristics differed markedly between cohorts. Mean BMI was significantly higher in the AHA cohort (32.4 kg/m², SD 10.6) versus ORCCA (25.6 kg/m², SD 4.7; p<0.001). Comorbidity prevalence was substantially higher in hospitalised patients: diabetes (10% vs 0.4%), hypertension (7% vs 0.6%), chronic kidney disease (2% vs 0%), and asthma (14% vs 8%), all p<0.01. However, 62% of hospitalised patients had no remarkable medical history. Initial symptoms in the AHA cohort more frequently included fever/chills (43%), cough (41%), shortness of breath (38%), nausea/vomiting/diarrhoea (32%), and chest pain (7%), while ORCCA athletes more often reported headache (23%), loss of smell/taste (23%), sore throat (18%), and nasal congestion (18%). Cardiac testing was far more common in the ORCCA cohort: ECG (95% vs 52%), troponin (87% vs 33%), echocardiogram (82% vs 7%), and cardiac MRI (14% vs 0.3%), all p<0.001. There were 12 deaths (2%) in the AHA cohort versus 0 in ORCCA. MACE occurred in 22 AHA patients (3.5%) and 22 ORCCA patients (0.6%). In the AHA cohort, MACE included ischaemic stroke (1%), pulmonary embolism (0.6%), new-onset heart failure (0.8%), sustained ventricular arrhythmias (0.5%), myocarditis (0.5%), and myocardial infarction (0.2%). In ORCCA, MACE consisted of 21 myocarditis cases (0.6%) and 1 pulmonary embolism (0.03%). Other severe clinical events occurred in 104 AHA patients (16.4%), including 60 (9%) requiring ventilation, 19 (3%) with shock, and 4 (0.6%) requiring haemodialysis/CRRT. Univariable analysis identified BMI as a significant predictor of death in the AHA cohort (OR 1.05, 95% CI 1.00–1.10; p=0.04). No significant predictors of MACE or other severe clinical events were identified. Median follow-up for ORCCA was 411 days (IQR 387–447); no follow-up was available for the AHA cohort beyond initial hospitalisation (median hospitalisation 4 days).
**Clinical Implications:** This study demonstrates that the risk of cardiac events in young adults aged 18–24 with COVID-19 is low overall, but hospitalised patients carry a substantially higher comorbidity burden and a 2% mortality rate. Elevated BMI is a significant predictor of death in hospitalised young adults, reinforcing obesity as a key modifiable risk factor. The stark contrast in cardiac testing rates between cohorts—low in hospitalised patients (only 7% received echocardiography, 0.2% cardiac MRI) versus high in athletes (82% echocardiography, 14% cardiac MRI)—raises concern for potential underdiagnosis of cardiac involvement in hospitalised patients and overdiagnosis in athletes. The study is limited by its comparison of two independent cohorts with different entry criteria, incomplete symptom documentation (27% missing in AHA), lack of follow-up beyond hospitalisation in the AHA cohort, and data collection prior to widespread COVID-19 vaccination. The relatively small number of events limits detailed risk factor modelling. These findings highlight the need for further research to identify risk factors for severe outcomes in young adults with COVID-19 and to establish appropriate cardiac testing protocols across the severity spectrum.