Fact Check-Study of Thai teenagers did not find one third experienced heart effects after COVID vaccination

A study of 301 teens in Thailand found mild and temporary heart rhythm changes after a second dose of the Pfizer-BioNTech COVID-19 vaccine among one in six teenagers, not one-third as social media posts claim. The study also saw possible signs of heart inflammation in just seven of those teens with rhythm changes and confirmed myocarditis in only one of the seven.

Social media users are circulating the study of post-vaccination heart effects in Thai teenagers with the claim that a third of participants experienced heart effects, and the suggestion that the results indicate a new danger level for children. These posts are missing context: the study’s authors concluded, “We found the risk of these symptoms to be not as low as reported elsewhere, but in all cases, symptoms were mild with full recovery within 14 days.”

A Twitter post shared more than 11,000 times (here) contains a link to the preprint study by Suyanee Mansanguan of Bhumibol Adulyadej Hospital in Bangkok and colleagues, with the comment, “BREAKING: A new study has found cardiovascular adverse effects in around a third of teens following Pfizer vaccination, and heart inflammation in one in 43, raising fresh concerns about the risks of vaccination for young people. This is beyond concerning.”

Similar versions of the post on Facebook can be seen (here) (here) (here) (here).

The original tweet garnered responses that include comments suggesting the study is a new cause for concern, such as “I am so worried about my teen-aged-children finding a healthy life partner in the future. My understanding is that ~61% of kids ages 12-17y.o. are (sorry for putting it bluntly) damaged goods now” (here) and “Makes me so angry that my very healthy son, a college athlete, was required to get this vaccine, or not continue his collegiate career” (here).

Mansanguan and colleagues note in their draft study, released as a preprint on Aug. 8 (here), that they likely saw higher rates of heart rhythm disturbance and signs of inflammation than in other studies because they did tests that detected mild changes in participants with no symptoms who would not ordinarily have been screened.

The analysis included 301 people aged 13-18 recruited from two Bangkok schools before receiving their second dose of the Pfizer-BioNTech mRNA COVID-19 vaccine in November and December of 2021. The students were mostly male (67%) and none had “abnormal” symptoms after their first vaccine shot, the study notes.

Before receiving the second vaccine shot, each participant had a physical exam, a heart ultrasound called an echocardiogram, heart rhythm measurements by electrocardiogram (ECG or EKG) and blood tests to look for heart-related biomarkers including Troponin-T and CK-MB, both markers of damage to heart muscle. The exam and the tests were repeated on days 3 and 7 following the second vaccine shot, and on day 14 for some of the teenagers. The participants also kept symptom diaries throughout the study period and were able to contact or visit the study team doctors at any time to ask questions or discuss unusual symptoms.

Overall, 50 of the 301 students reported fever after the second vaccine shot and 35 reported headache, both common general side effects following COVID-19 vaccination.

Among cardiovascular effects detected only by ECG, 54 participants (18%, so roughly one in six, not one in three as social media rates) had rapid heartrate or abnormal heart rhythm. Of these, 39 had reported symptoms such as palpitations or chest pain. Fifteen reported no symptoms at all.

Among the participants with abnormal ECG, seven – all males -- also had elevated biomarkers of heart muscle injury or inflammation. Of these seven, four had reported chest discomfort or pain, but three had no symptoms other than the elevated biomarkers. All seven also had normal heart function and no sign of reduced pumping ability that can signal heart failure.

One young man was admitted to the hospital intensive care unit for observation of his arrhythmia over four days, treated with anti-inflammatory drugs, and his symptoms resolved within days, with no detectable damage to his heart, according to the report.

Of that patient -- the only one formally diagnosed with myocarditis -- the study authors write, “One patient with myopericarditis in our study follow-up with [cardiac MRI] at 5 months after vaccination showed complete recovery and no scar.”

Although the study authors note in their paper that many of the survey participants (44%) had other underlying diseases including asthma, allergies, blood or thyroid disorders and migraine, the study does not analyze whether these conditions were associated with differences in risk for side effects or cardiovascular effects after the vaccine. The authors also note that they were unable to do baseline testing of kids prior to the first vaccine shot, which is a limitation of the study. Reuters contacted the study’s senior author for comment.

The rates of heart effects or suspected heart effects detected by prospectively testing everyone in the Thai study are higher than seen in many studies that rely on voluntary reporting of possible adverse events to databases such as the U.S. Vaccine Adverse Event Reporting System (VAERS) (, or studies that rely on retrospective analysis of medical records for people diagnosed with myocarditis.

The Thai team confirmed one case of myocarditis (heart inflammation) in the 301 students -- not one in 43 as suggested in social media posts.

The 1 in 301 myocarditis rate in the Thai study would translate to roughly 332 per 100,000. In contrast, one retrospective study in Israel (here) found that the odds of myocarditis following vaccination were nearly twice as high after the second shot than after the first. The rates were also highest among young men aged 16-19, at 13.60 per 100,000.

A recent study by CDC researchers looking at data from 40 U.S. health systems found that myocarditis following COVID-19 vaccination was diagnosed at a rate of 22.0–35.9 per 100,000 among males aged 12-17 (here).

However, a U.S. pediatric cardiologist who reviewed the Thai study at (here) questioned whether the heart readings that the study calls abnormal really were indicative of adverse effects on the heart, especially in teenagers with no other symptoms.

Abnormal ECG alone is not sufficient to diagnose myocarditis in someone without symptoms, writes Dr. Eric Han, who notes that the reader also cannot tell how abnormal any readings might have been because data from before vaccination isn’t provided for comparison.

Of the different types of heart rhythm described as abnormal in the study, all but one could be considered normal in a child depending on the circumstances, Han notes. “Elevated troponin has its own causes as well, not all of which are myocarditis,” he also writes.

“To the trained observer,” Han concludes, “there are no shocking findings in this study. Overall, it supports the current body of knowledge regarding COVID vaccination myocarditis.”


False. The study of teenagers in Thailand following a second COVID-19 vaccination found that 18% -- not one third -- experienced any detectable cardiac effect, and that 1 in 301, not 1 in 43, had confirmed myocarditis. A large proportion of purported abnormalities detected by testing were without symptoms, and 100% of the teens in the study fully recovered after 14 days, the authors reported.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here .