Japanese researchers said they found evidence of long-term heart damage in people who received COVID-19 vaccines — including in asymptomatic patients — even though vaccine-induced myocarditis was thought to be rare, transient and limited to subjects experiencing heart symptoms.

Regardless of age or sex, patients who received their second vaccination up to 180 days before imaging showed a 47% higher uptake in heart tissues of fluorine-18 fluorodeoxyglucose (FDG), an imaging agent, than unvaccinated subjects.

FDG is identical to glucose, a sugar that is the body’s main energy source, but it contains fluorine-18, a radioactive form of fluorine that allows imaging of organs and tissues where FDG accumulates.

Stressed or damaged cells, a hallmark of myocarditis, take up more glucose than healthy cells.

Researchers led by Takehiro Nakahara at Keio University School of Medicine used a retrospective study design to compare positron emission tomography/computed tomography (PET/CT) scans between patients undergoing imaging before COVID-19 vaccines were available (from Nov. 1, 2020, to Feb. 16, 2021) to scans on other subjects after the vaccine rollout (Feb. 17, 2021, to March 31, 2022).

The 1,003 subjects — 700 vaccinated against SARS-CoV-2 and 303 unvaccinated — were grouped by age (younger than 40, 41-60 years, and older than 60), sex and time between vaccination and PET/CT.

Of the vaccinated subjects, 78% received the Pfizer-BioNTech BNT162b2 product while 21% got the Moderna mRNA shot. No difference in FDG uptake was observed in patients taking either product.

Subjects who received AstraZeneca’s shot, or one of the other less common vaccines, were excluded because their low numbers would have introduced uncertainty to the analysis.

To capture safety signals from asymptomatic subjects only, investigators chose subjects who were scanned for issues unrelated to the heart. Most scans were for cancer diagnoses.

Higher FDG uptake was also observed in tissues outside the heart, including the liver, spleen, and the whole body, and particularly in axial (armpit) lymph nodes. Earlier studies claimed these effects disappear after 2-3 weeks, but Nakahara showed they last for up to six months.

Twenty-five subjects had more than one scan during one or both study periods, and 16 underwent a PET/CT test in both the pre- and post-vaccine time periods. Within this small subgroup, vaccinated subjects showed significantly higher FDG uptake in both the heart and axial lymph nodes.

Although myocarditis persisted longer than 120 days, its occurrence was not statistically significant beyond that time point.

Myocarditis occurs in the general population at rates of 6.1 and 4.4 per 100,000 for men and women, respectively. Symptoms include chest pain, shortness of breath and heart palpitations.

According to the Centers for Disease Control and Prevention (CDC), “Most patients with myocarditis or pericarditis after COVID-19 vaccination responded well to medicine and rest and felt better quickly.”

Treatment for myocarditis involves rest, pain relievers, anti-inflammatory medications and in some cases, hospitalization.

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