I have now found the UK government advice on Myocarditis and Pericarditis after covid vaccination - I searched via a search engine after missing it on the govt website.
https://www.gov.uk/government/publi...ination-guidance-for-healthcare-professionals
This was updated on December 7th and says
"In those aged under 18 years, the reported rate for heart inflammation (myocarditis and pericarditis) is 10 per million doses (first dose or unknown dose) of the Pfizer vaccine."
This is apparently for all cases of reported adverse reaction.
The two US studies it references are this,
https://jamanetwork.com/journals/jamacardiology/fullarticle/2783052
from August,
and this,
https://publications.aap.org/pediat...ated-Myocarditis-in?autologincheck=redirected
from November.
The November study involves 63 adolescents hospitalised following Pfizer or Moderna vaccination, so I did some arithmetic.
Official statistics for US vaccination rates give this,
"As of July 16, 2021, approximately 8.9 million
U.S.
adolescents aged 12-17 years had received Pfizer-BioNTech vaccine."
- from
https://www.cdc.gov/mmwr/volumes/70/wr/mm7031e1.htm
So I have divided 8.9 million by 63 (which is not necessarily a total figure, but a figure from 1 study) and I get very close to 1 hospitalised case per 141 thousand adolescents vaccinated.