Fact Check-'Very low’ number of under-18s with myocarditis after COVID-19 vaccine

Social media users have shared a snippet of a single bullet point from COVID-19 vaccine guidance published by the UK Health Security Agency (UKHSA), which misleads on the contents of the 1,400-word report.

Published on Nov. 29, the guidance offers clinical advice to detect and respond to potential cases of myocarditis and pericarditis, an inflammation of the heart, after vaccination.

The UKHSA says in the first 20 lines of the text that the condition has been observed in people who received the mRNA vaccines produced by Pfizer/BioNTech and Moderna – but makes clear that it’s a rare condition, which is usually mild and stable.

However, several social media posts have shared a select portion of one line in the text, which appears to suggest the condition is common in children.

“’..significant left ventricular fibrosis has been described in a high percentage of children’,” wrote one Twitter user who was retweeted dozens of times (here).

Other posts have quoted the UKHSA report in a similar fashion (here, here, here).

This is misleading, as the full line reads: “Myocarditis – significant left ventricular (LV) fibrosis has been described in a high percentage of children admitted to hospital, with a small percentage of these having non-sustained ventricular tachycardia (VT).”

The same point also comes directly after a line that says cases are mostly mild and that the majority of people recover in full without needing medical treatment.

Reuters asked the UKHSA for comment. It said the line quoted did not suggest there is a high number of children who develop myocarditis after vaccination.

There are around 10 reports of myocarditis and pericarditis in under-18s per 1 million doses following the Pfizer vaccine (Moderna is not currently available for this age group), the agency added.

Commenting specifically on the bullet point used selectively by social media users, the UKHSA told Reuters that it referenced one study in the United States which reports that, of the very small number of cases of myocarditis following vaccination in under-18s, a high percentage have had a particular type of clinical feature, known as left ventricular fibrosis.

Though the study refers to a ‘high percentage’, the total numbers will still be very low, it said.

The UKHSA explained that, for the population as a whole, myocarditis following COVID-19 vaccination typically resolves within a short time and most cases respond well to treatment.

Available data also shows no major complications in the months following, it added.

Reuters also presented the claims to the Medicines and Healthcare products Regulatory Agency (MHRA), the British regulator for drugs and vaccines.

It directed Reuters to the following section of its latest summary of “Yellow Card” reports - potential vaccine side effects which can be submitted by any member of the public (here).

The MHRA said myocarditis is a “recognised potential risk” with the mRNA COVID-19 vaccines but that reports of myocarditis and pericarditis are “very rare” within the under-18 cohort.

MHRA chief executive Dr June Raine told Reuters: “The MHRA closely monitors the safety and effectiveness of the COVID-19 vaccines, including reports of suspected heart inflammation, myocarditis or pericarditis.

“Myocarditis or pericarditis remains a very rare potential risk with the Pfizer and Moderna vaccines, and the events reported are typically mild with individuals usually recovering within a short time with standard treatment and rest.

“The public’s safety is our top priority. Our advice remains that the benefits of the vaccine outweigh the risks in the vast majority of people. It is vitally important that people come forward for their vaccination and for their booster doses when invited to do so.

“We ask anyone who suspects they have experienced a side effect linked with their COVID-19 vaccine to report it to the Coronavirus Yellow Card website.”

Reuters previously debunked claims that myocarditis following the COVID-19 vaccine is irreversible (here).


Misleading. UKHSA guidance does not suggest there are high numbers of children who get myocarditis after vaccination. The overall reporting rate of myocarditis and pericarditis in under-18s is very small and most cases are mild and self-limiting.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here.