Very rarely; mostly mild heart-muscle inflammation

Cardiomyopathy is a long-term disease of the heart muscle, and myocarditis is a separate condition: acute inflammation that can occasionally lead to cardiomyopathy. Large population studies and surveillance systems confirm that mRNA vaccines cause rare myocarditis. They have not identified an increase in cardiomyopathy diagnoses linked to vaccination. Long-term follow-up is still limited to a few years, so researchers continue to monitor myocarditis survivors. Current evidence does not support a causal link to cardiomyopathy. Anyone with chest pain, shortness of breath or palpitations after vaccination should seek medical care.

• Myocarditis and pericarditis are recognized rare side effects of mRNA COVID-19 vaccines, with the highest risk in males aged about 12 to 29.
• Reported rates in the highest-risk group are roughly 1 in 5,000 to 1 in 20,000 doses, and much lower across the whole population.
• Most vaccine-associated myocarditis cases are mild, and most patients recover within weeks to months.
• Follow-up studies of some patients have found MRI scar-like changes, but they have not shown a rise in cardiomyopathy or heart failure.
• COVID-19 infection itself carries a higher risk of myocarditis than vaccination, and it can also cause other heart complications.

The important distinction is between the established, rare risk of myocarditis and its much rarer possible consequence, cardiomyopathy. Most vaccine-associated myocarditis patients improve, although some have persistent heart-muscle abnormalities whose long-term significance remains uncertain. Evidence for myocarditis causation is strong; evidence defining the exact frequency of subsequent cardiomyopathy is limited, so the observed Hong Kong case should not be treated as a precise worldwide risk estimate.

• The causal relationship between Pfizer and Moderna COVID vaccines and myocarditis is well established; myocarditis means inflammation of the heart muscle.
• Myocarditis can sometimes progress to dilated cardiomyopathy, in which the heart becomes enlarged and pumps less effectively. That progression is not inevitable.
• In the Hong Kong study, dilated cardiomyopathy occurred in 1 of 104 postvaccination myocarditis patients during six months of follow-up—not in 1% of vaccine recipients.
• For the 2023–2024 mRNA formulations, myocarditis or pericarditis was recorded within seven days at approximately 8 cases per million doses overall among people aged 6 months–64 years, and 27 per million among males aged 12–24. These are not cardiomyopathy rates.
• A French nationwide study found fewer serious cardiovascular complications after vaccine-associated myocarditis than after myocarditis associated with COVID infection or other causes.

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The question names cardiomyopathy, but the harm the evidence actually establishes is myocarditis (and pericarditis), so the answer is given for that. Multiple large population studies and health agencies worldwide agree that mRNA COVID vaccines causally raise the risk of heart-muscle inflammation, almost always within days of a second dose and mostly in young males. The risk is small in absolute terms, well below the risk of heart complications from COVID-19 itself, and most cases are mild and resolve. Cardiomyopathy — a broader, often chronic heart-muscle disorder — can occasionally follow myocarditis, and rare cases have been reported after vaccination, but it is not established as a direct vaccine effect.

• Large studies in Israel, the Nordic countries, and the US found a real but rare rise in myocarditis and pericarditis after mRNA COVID vaccines, strongest after the second dose in males aged about 16-29.
• Overall rates are on the order of 1 to 4 cases per 100,000 vaccinated people, with the highest rates in young males (roughly 1 in 5,000 to 1 in 15,000 after a second dose).
• Most reported cases are mild and settle with rest and standard treatment; follow-up studies find low rates of serious ongoing heart problems.
• Cardiomyopathy — chronic disease of the heart muscle — is a different diagnosis from myocarditis and has only been reported rarely after vaccination, usually following myocarditis rather than as a direct effect.
• The risk of myocarditis is higher after COVID-19 infection itself than after vaccination, which is why health agencies still recommend the vaccines.

The scientific evidence shows that COVID-19 mRNA vaccines are associated with a very rare risk of myocarditis, particularly in young males. The incidence is estimated at 0.89–5 cases per 100,000 vaccinated people, or 8–27 cases per million doses, depending on the study and population. Most cases are mild and resolve with minimal treatment, but some individuals experience long-term cardiac effects such as fibrosis or arrhythmias. Cardiomyopathy, a more chronic and structural heart muscle disease, is not directly linked to the vaccine by current evidence. However, myocarditis can sometimes lead to or be misdiagnosed as cardiomyopathy, especially if the inflammation causes lasting damage. The FDA and CDC have updated vaccine labeling to include warnings about myocarditis and pericarditis, but not cardiomyopathy. The overall risk remains very low, and the benefits of vaccination in preventing severe COVID-19 and its complications are well-documented.

• COVID-19 mRNA vaccines are linked to myocarditis, especially in males aged 12–24, at a rate of 8–27 cases per million doses.
• Myocarditis after vaccination is usually mild and self-limiting, but some cases result in long-term cardiac issues like fibrosis or arrhythmias.
• The incidence of vaccine-associated myocarditis is estimated at 0.89–5 cases per 100,000 vaccinated people.
• No strong evidence links COVID-19 vaccines directly to cardiomyopathy, but myocarditis can sometimes progress to or be misdiagnosed as cardiomyopathy.
• The FDA and CDC continue to monitor and update warnings about myocarditis and pericarditis following mRNA COVID-19 vaccination.

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