Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Federal Ministry of Education and Research. Background/Introduction To increase vaccine safety, adverse events of COVID-19 vaccines are continuously monitored. This also includes rare complications of the cardiovascular system. However, it has so far been scarcely investigated whether chronic and subtle, clinically inapparent myocardial and/or vascular injury occurs in a COVID-19 vaccinated population on a broader basis. Purpose The aim of this study was to analyse chronic and subclinical adverse effects of COVID-19 vaccines on the cardiovascular system. Vaccine-associated myocardial involvement was examined by measuring high-sensitive troponin T (hsTnT) while mid-regional pro-adrenomedullin (MR-proADM) levels were evaluated to assess endothelial dysfunction. Methods This was a prospective observational study with a vulnerable population of healthcare workers (HCWs) and elderly (> 70 years) patients who were vaccinated with either the ChAdOx1 nCov-19 adenoviral vector vaccine (AZ) and the BNT162b2 mRNA vaccine (BNT), or with BNT/BNT between the 12th of January and 30th of November 2021 (Figure 1). HsTnT and MR-proADM was measured in blood samples at three visits (V1: 1st baseline, immediately before vaccination; V2: 3-4 weeks after 1st vaccination; V3: 3-4 weeks after 2nd vaccination). HsTnT levels were further compared with a healthy reference cohort. Results 120 participants were included (V1=119, V2=120, V3=120). 50 (41.7%) received AZ/BNT. 70 (58.3%) were vaccinated with BNT/BNT of whom 20 (16.7%) were elderly and 50 (41.7%) were HCWs. Median hsTnT levels were at 4ng/L, 5ng/L and 4ng/L (V1-V3) for AZ/BNT and at 6ng/L, 6ng/L and 6ng/L (V1-V3) for BNT/BNT, respectively (Figure 2). Levels of MR-proADM were 0.43nmol/L, 0.46nmol/L and 0.44nmol/L (V1-V3) in the AZ/BNT cohort and 0.49nmol/L, 0.43nmol/L and 0.49nmol/L in the BNT/BNT group. Change of hsTnT and MR-proADM values between visits did not show significant increases. Two cases had a transient (7ng/L, 19ng/L, 5ng/L; male, 29 years) or permanent (15ng/L, 21ng/L, 25ng/L; male, 37 years) increase in hsTnT levels, exceeding the upper reference limit (≥ 14ng/L). Compared to the reference population (n=300), median hsTnT was significantly higher at all visits for both vaccination groups (p<0.05), without significant differences between the AZ/BNT and BNT/BNT cohorts. Conclusion(s) With two individual exceptions, no overall chronic myocardial injury or endothelial involvement was observed up to 4 weeks after the 2nd SARS-CoV-2 vaccination when comparing hsTnT and MR-proADM after both vaccinations to baseline values.Figure 1Vaccination scheme.Figure 2.hsTnT values at V1-V3.