We came across this post on Twitter:

The tweet, with over 11,000 retweets, contains a link to a preprint study by Suyanee Mansanguan of Bhumibol Adulyadej Hospital in Bangkok and colleagues. When we took a look at the study, we found that it utilised a sample of 301 people aged 13-18 recruited from two Bangkok schools before receiving their second dose of the Pfizer-BioNTech mRNA COVID-19 vaccine. The students had no “abnormal” symptoms after their first vaccine shot.

Before receiving the second vaccine shot, each participant received a physical exam, a heart ultrasound, heart rhythm measurements and blood tests to look for heart-related biomarkers including Troponin-T and CK-MB, both markers of damage to heart muscle. The tests were repeated on days 3 and 7 following the second vaccine shot. The participants were also able to contact or visit the doctors who conducted the study at any time to discuss potential symptoms they encountered.

Ultimately, the study found that for cardiovascular effects detected only by ECG, 54 participants (18%, or about one in six, not one in three as described by the tweet) had rapid heartrate or abnormal heart rhythm. Of these, 39 had reported symptoms such as palpitations or chest pain. 15 reported no symptoms at all.

For the more serious cardiovascular adverse effects, one case of myopericarditis, four cases of subclinical myocarditis, and two cases of pericarditis among the 301 participants, however each case had mild symptoms. The study concluded by stating that the long-term outcomes of COVID-19 vaccine have not been described, but nearly 100% of patients with symptoms had recovered within 1–2 weeks, with the one patient found with myopericarditis showing a complete recovery 5 months after vaccination.

Based on this, the claim that a third of teens in the study had cardiovascular adverse effects and one in 43 had suffered from heart inflammation is 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.