The CDC has confirmed that heart inflammation or myocarditis can occur after receipt of mRNA COVID vaccines.

What is the mechanism?

This study found mRNA-generated spike protein in biopsies of some of those patients’ hearts:

Intramyocardial Inflammation after COVID-19 Vaccination: An Endomyocardial Biopsy-Proven Case Series, International Journal of Molecular Sciences, 22 June 2022

Myocarditis in response to COVID-19 vaccination has been reported since early 2021. In particular, young male individuals have been identified to exhibit an increased risk of myocardial inflammation following the administration of mRNA-based vaccines.

Here, we present a comprehensive histopathological analysis of endomyocardial biopsies (EMBs) from 15 patients with reduced ejection fraction (LVEF = 30 (14-39)%) and the clinical suspicion of myocarditis following vaccination with Comirnaty® (Pfizer-BioNTech) (n = 11), Vaxzevria® (AstraZenica) (n = 2) and Janssen® (Johnson & Johnson) (n = 2).
…
Immunohistochemical EMB analyses reveal myocardial inflammation in 14 of 15 patients.
…
Importantly, infectious causes have been excluded in all patients.
…
The SARS-CoV-2 spike protein has been detected sparsely on cardiomyocytes of nine patients, and differential analysis of inflammatory markers such as CD4+ and CD8+ T cells suggests that the inflammatory response triggered by the vaccine may be of autoimmunological origin. Although a definitive causal relationship between COVID-19 vaccination and the occurrence of myocardial inflammation cannot be demonstrated in this study, data suggest a temporal connection. The expression of SARS-CoV-2 spike protein within the heart and the dominance of CD4+ lymphocytic infiltrates indicate an autoimmunological response to the vaccination.

How did spike protein get into their hearts? CDC said spike was produced locally (in the arm) and removed by the body in a week or two.

The spike protein was found in sparse cells (cardiomyocytes) in 9 of 15 cases (Figure 2).

Autoimmunity a factor?

Because viral infections have been ruled out as the cause for myocarditis/myocardial inflammation, autoimmunological mechanisms might be an explanation.

Why is the body attacking itself (autoimmunity)?

Cross-reactivity of spike protein antibodies with myocardial contractile proteins, mRNA immune reactivity and hormonal involvement, have been discussed as potential mechanisms by which COVID-19 mRNA vaccines induce hyperimmunity [20].

Spike protein has been found in post-vaccine shingles lesions and now the heart.

Thus, vaccine-encoded spike protein seems to reach the heart, where it might trigger an inflammatory response, resulting in the development of myocarditis.