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This study aims to develop and validate the Post-COVID Symptom Scale for Children/Youth (PCSS-C/Y), which is a comprehensive tool for measuring the symptom burden of post-COVID-19 conditions-persistent symptoms after SARS-CoV-2 infection, commonly known as Long COVID-and its impact on health-related quality of life among children and adolescents. Parents of children and adolescents, adolescents, and young adults with and without a history of COVID-19 were invited to fill in a questionnaire from October 2022 to June 2023. There were 386 valid parent proxy-reported responses, 433 valid adolescent self-reported responses, and 324 valid young adult self-reported responses included in the final analysis. The PCSS-C/Y demonstrated stable factor structure and good internal consistency in different sampling groups. The scale score was negatively associated with Paediatric Quality of Life Inventory (PedsQL) scores (young adult self-report, adjusted R 2  = 0.394; adolescent self-report, adjusted R 2  = 0.219; parent-report, adjusted R 2  = 0.292), while it was positively associated with Strengths and Difficulties Questionnaire (SDQ) scores (young adult self-report, adjusted R 2  = 0.195; adolescent self-report, adjusted R 2  = 0.154; parent-report, adjusted R 2  = 0.239). The scale can also discriminate the post-infected cases and control cases, Cohen's d = 0.41, 0.50, and 0.38 for adult self-report, adolescent self-report, and parent-report, respectively. Conclusions: The PCSS-C/Y is a valid and reliable tool for quantifying the diverse symptomatology of post-COVID-19 conditions in children and adolescents. It provides quantifiable measurements that enable clinicians to monitor post-COVID-19 symptoms in children and young people and facilitates the development of interventions for post-COVID-19 conditions., Competing Interests: Declarations. Ethical approval: The study is approved by the ethics committee of the Institutional Review Board of the Hong Kong University/Hospital Authority Hong Kong West Cluster and Hospital Authority Central Institutional Review Board (Central IRB) (UW 20–292, UW 20–177, HKCH-REC-2022–009). Consent to participate: Informed consent was obtained from all participants in this study. Conflict of interest: The authors declare no competing interests., (© 2024. The Author(s).)

Background: Vaccine-related acute myocarditis is recognized as a rare and specific vaccine complication following mRNA-based COVID-19 vaccinations. The precise mechanisms remain unclear. We hypothesized that natural killer (NK) cells play a central role in its pathogenesis., Methods: Samples from 60 adolescents with vaccine-related myocarditis were analyzed, including pro-inflammatory cytokines, cardiac troponin T, genotyping, and immunophenotyping of the corresponding activation subsets of NK cells, monocytes, and T cells. Results were compared with samples from 10 vaccinated individuals without myocarditis and 10 healthy controls., Findings: Phenotypically, high levels of serum cytokines pivotal for NK cells, including interleukin-1β (IL-1β), interferon α2 (IFN-α2), IL-12, and IFN-γ, were observed in post-vaccination patients with myocarditis, who also had high percentage of CD57 + NK cells in blood, which in turn correlated positively with elevated levels of cardiac troponin T. Abundance of the CD57 + NK subset was particularly prominent in males and in those after the second dose of vaccination. Genotypically, killer cell immunoglobulin-like receptor (KIR) KIR2DL5B(-)/KIR2DS3(+)/KIR2DS5(-)/KIR2DS4del(+) was a risk haplotype, in addition to single-nucleotide polymorphisms related to the NK cell-specific expression quantitative trait loci DNAM-1 and FuT11, which also correlated with cardiac troponin T levels in post-vaccination patients with myocarditis., Conclusion: Collectively, these data suggest that NK cell activation by mRNA COVID-19 vaccine contributed to the pathogenesis of acute myocarditis in genetically and epidemiologically vulnerable subjects., Funding: This work was funded by the Hong Kong Collaborative Research Fund (CRF) 2020/21 and the CRF Coronavirus and Novel Infectious Diseases Research Exercises (reference no. C7149-20G)., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

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.)

Competing Interests: As an editor of the journal, JC Yam was not involved in the peer review process. Other authors have disclosed no conflicts of interest.