mRNA is not found in vaccines. The first mRNA vaccine was used in ebola. Tests were being run on a flu vaccine but then Covid hit. So really Covid is the first wide spread vaccine to use mRNA technology.
The technology behind mRNA has been in development for 30 years
mRNA is not the cause of the Afib its the immune response that causes the Afib. In the US 613 million Covid shots have been given. Worldwide 12.7 Billion shots have been given.
https://www.idsociety.org/covid-19-real-time-learning-network/vaccines/mrna-vaccines-what-to-know/#/+/0/publishedDate_na_dt/desc/
COVID-19 is much more likely to cause heart damage than the vaccine, especially among older people with medical conditions that put them at higher risk for a serious infection.
Some people have concerns that mRNA vaccines cause heart problems. Early in the pandemic, there were rare instances of teenage boys and younger men experiencing heart inflammation, which happened about as frequently as being struck by lightning (about 1 in 10,000 vaccinations).
Most of these people responded well to medicine and rest and felt better quickly.
This small risk is even less now that we have safety recommendations like spacing out vaccines for young men and women. COVID-19 is much more likely to cause heart damage than the vaccine, especially among older people with medical conditions that put them at higher risk for a serious infection.