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Background: Age-specific incidence of acute myocarditis/pericarditis in adolescents following Comirnaty vaccination in Asia is lacking. This study aimed to study the clinical characteristics and incidence of acute myocarditis/pericarditis among Hong Kong adolescents following Comirnaty vaccination., Methods: This is a population cohort study in Hong Kong that monitored adverse events following immunization through a pharmacovigilance system for coronavirus disease 2019 (COVID-19) vaccines. All adolescents aged between 12 and 17 years following Comirnaty vaccination were monitored under the COVID-19 vaccine adverse event response and evaluation program. The clinical characteristics and overall incidence of acute myocarditis/pericarditis in adolescents following Comirnaty vaccination were analyzed., Results: Between 14 June 2021 and 4 September 2021, 33 Chinese adolescents who developed acute myocarditis/pericarditis following Comirnaty vaccination were identified. In total, 29 (87.88%) were male and 4 (12.12%) were female, with a median age of 15.25 years. And 27 (81.82%) and 6 (18.18%) cases developed acute myocarditis/pericarditis after receiving the second and first dose, respectively. All cases are mild and required only conservative management. The overall incidence of acute myocarditis/pericarditis was 18.52 (95% confidence interval [CI], 11.67-29.01) per 100 000 persons vaccinated. The incidence after the first and second doses were 3.37 (95% CI, 1.12-9.51) and 21.22 (95% CI, 13.78-32.28 per 100 000 persons vaccinated, respectively. Among male adolescents, the incidence after the first and second doses were 5.57 (95% CI, 2.38-12.53) and 37.32 (95% CI, 26.98-51.25) per 100 000 persons vaccinated., Conclusions: There is a significant increase in the risk of acute myocarditis/pericarditis following Comirnaty vaccination among Chinese male adolescents, especially after the second dose., Competing Interests: Potential conflict of interest. C. C. has received grants outside of the submitted work from the Food and Health Bureau of the Hong Kong Government, Hong Kong Research Grants Council, Hong Kong Innovation and Technology Commission, Pfizer, IQVIA, and Amgen; and a personal fee from Primevigilance Ltd. A. S. Y. L. received grants outside of the submitted work from the Health and Medical Research Fund, Food and Health Bureau of the Hong Kong Government Special Administrative Region. M. Y. N. has received funding/education grants from the Food and Health Bureau of the Hong Kong Government, Radiological Society of North America, GE, Lode, Arterys, Bayer, Circle Cardiovascular Imaging and TeraRecon; honoraria for education activities from Boehringer Ingelheim; reports the following leadership roles: Vice Chair of the Education Committee for Society of Cardiovascular Magnetic Resonance and Member of the Corporate Relations Committee for Society of Cardiovascular Computed Tomography. G. C. F. C. is the CMO of Xellera and advisor of Pangenia. Y. L. L. received Government funding for COVID-19 Vaccinations in Adolescents (COVA) and is the Chairman of the Scientific Committee on Vaccine Preventable Diseases, Centre for Health Protection, HKSAR. I. W. has received research funding outside of the submitted work from Amgen, Bristol-Myers Squibb, Pfizer, Janssen, Bayer, GSK, Novartis, Hong Kong Research Grants Council, Hong Kong Health and Medical Research Fund, National Institute for Health Research in England, European Commission, and National Health and Medical Research Council in Australia (Research grants on pharmacoepidemiology to The University of Hong Kong outside of the submitted work); consultancy fee for advising IQVIA on pharmacoepidemiology studies outside of the submitted work; payment for expert testimony from Appeal Court in Hong Kong (expert report on effects of cannabis outside of the submitted work); and speaker fees from Janssen and Medicine in the previous 3 years; reports the following leadership roles: Member of Pharmacy and Poisons Board (this is the regulatory agency in pharmaceutical product licensing), Member of the Expert Committee on Clinical Events Assessment Following COVID-19 Immunization (advise the Hong Kong Government on safety of COVID-19 vaccines), and Member of the Advisory Panel on COVID-19 Vaccines of the Hong Kong Government (advise the Hong Kong Government on the emergency use of COVID-19 vaccines). He is also an independent nonexecutive director of Jacobson Medical in Hong Kong (salaried). P. I. has received grants outside of the submitted work from the Food and Health Bureau of the Hong Kong Government, Hong Kong Research Grants Council, and Hong Kong Jockey Club Charities Trust. M. T. Y. L. reports receiving Honorarium for a talk on ADHD. W. K. C. N. reports personal honoraria for Guerbet online lecture on pediatric cardiac imaging; holds 100 shares in Moderna stock, 50 shares in Biotech stock since April, owned 100 shares in Pfizer stock from July 2020 to January 2021. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2021. Published by Oxford University Press for the Infectious Diseases Society of America.)