TLDR:<p>Japanese study (data through Dec 2023) found mRNA COVID vaccines linked to increased heart inflammation risk - Moderna showed higher risk (54x) than Pfizer (15.6x). Out of 880,999 reports, there were 919 myocarditis and 321 pericarditis cases. Most affected were males under 30, occurring within 8 days post-vaccine. While most patients recovered, some cases were severe or fatal.