The study cited is from Hong Kong:
https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.123.064772
The main vaccines used in China were CoronaVac and Sinopharm, which are not mRNA vaccines like Moderna and Pfizer, but instead use inactivated virus. mRNA vaccines were not approved by China until May 2023.
However, they did a test in Hong Kong when there was an Omicron surge in January of 2022 wherein half the population received CoronaVac and half Pfizer (object of the test being to find out whether there were any differences in efficacy and duration of efficacy):
https://pubmed.ncbi.nlm.nih.gov/35850128/
See also (sorry it’s NPR and you have to read past their pontificating — they really are insufferable):
The portion of the study in Circulation we can read without paying for access does not specify when these kids were vaccinated, when diagnosed with vaccine-related myocarditis, or which vaccines they received. These would be interesting things to know to develop a better picture. Of course the incidence rate of vaccine-related myocarditis for each group would also be of interest.
If any of you have free access to the Circulation article in question (first !ink in my #16) and can answer the questions posed once in said #16, it would be much appreciated.