Moderna booster vaccine trials didn’t cause “mass heart attacks” in children, contrary to online claims

Key takeaway

Moderna’s KidCOVE study evaluated the safety and effectiveness of COVID-19 vaccines in almost 12,000 children for over a year each. One child who had received a booster dose died during the follow-up period for reasons unrelated to the vaccine. This isn’t unusual based on the death rates for children this age and the large number participating in the trial.

Reviewed content

Verdict:

Inaccurate

Claim:

“Moderna Covered Up ‘Mass Heart Attacks of Toddlers’ in COVID Jab Trials”

Verdict detail

Factually inaccurate: Moderna published results of the KidCOVE trial showing that one child died during the study. Contrary to the claim, this didn’t happen more than once.
Lacks context: The news articles and social media posts didn’t explain that the child who died was one of almost 12,000 who took part in the trial. This is important to understand because a death wouldn’t be statistically unexpected in a group this large.

See our method of evaluation

Full Claim

“Moderna Covered Up ‘Mass Heart Attacks of Toddlers’ in COVID Jab Trials”

The development and wider rollout of the COVID-19 vaccines involved dozens of trials to check the safety and effectiveness across different vaccines, doses, timeframes, populations, and virus variants. An important aspect of these trials is to determine what side effects are caused by the vaccines, which may require recruiting thousands of people.

On 8 January 2025, the website The People’s Voice (formerly NewsPunch) published an article with the headline “Moderna Covered Up ‘Mass Heart Attacks of Toddlers’ in COVID Jab Trials”. Social media posts featuring this headline were shared over 3,000 times on Facebook. As this review will explain, this claim is factually incorrect and misleading.

Media Bias/Fact Check describes The People’s Voice as having low credibility and a history of promoting “extreme right-wing conspiracy theories and pseudoscience misinformation”. Science Feedback has previously reviewed several inaccurate claims from The People’s Voice.

One death occurred during the trial, which was unrelated to the vaccine

The KidCOVE study tested the safety and effectiveness of Moderna’s COVID-19 vaccine (referred to as mRNA-1273) in children between the ages of 6 months and 11 years. The trial began in March 2021 and it expanded to include multiple groups testing different dosages and booster doses. Almost 12,000 children spent up to 17 months taking part in the trial.

In 2022, the trial began to test a booster dose of an updated vaccine named mRNA-1273.214. This booster vaccine was designed to target the original SARS-CoV-2 virus as well as the newly circulating Omicron variant. This new booster had already been tested in adults, showing that it improved the immune response against Omicron and leading to Emergency Use Authorization by the U.S. Food and Drug Administration^[1].

One group of more than 2,200 children aged six months to five years received the new booster vaccine, having already received two doses of the standard vaccine. During the follow-up period, one of the children in this group died of cardio-respiratory arrest. This death was included in data reported by Moderna to the EU Clinical Trials Register in September 2024.

Contrary to the headline from The People’s Voice and the Facebook posts, there were no other deaths reported in the trial data.

This death is classified in the trial data as “Adverse event, serious fatal”; however, this doesn’t mean that the vaccine caused it. Adverse events are often misrepresented in misleading vaccine safety claims. Adverse events aren’t the same as side effects—they are any negative occurrences that happen during a trial, and on their own don’t indicate causality. In fact, the data report from Moderna explicitly says that the death in the KidCOVE trial wasn’t causally related to treatment.

At the time of this review’s publication, Moderna hasn’t released further information about the death or the results of this part of the KidCOVE trial. The trial concluded in 2024 and the researchers have published detailed articles on different parts of the trial^[2,3]. This included a study on the safety and immune response to the standard booster in children aged six months to 11 years^[4]. However, detailed results of the group receiving the newer bivalent booster, including the child who died, haven’t been published yet.

Over a million real-world vaccinations haven’t raised safety concerns

Beyond the safety data from Moderna’s KidCOVE studies^[2-4], the vaccine’s safety in young children continued to be monitored after receiving Emergency Use Authorization

In 2022, the U.S. Centers for Disease Control and Prevention (CDC) reviewed initial vaccine safety data after approximately one million children aged six months to five years old received mRNA COVID-19 vaccines (both Moderna and Pfizer-BioNTech). Among the serious adverse events reported since the vaccines were authorized, one case of febrile seizure was “plausibly associated with vaccination”. However, overall it found that “no unexpected safety concerns were detected”.

A 2023 report by the CDC reviewed initial safety data for booster vaccination for 550,000 children aged six months to five years old. These booster doses were predominantly Pfizer-BioNTech vaccines with a minority from Moderna. This analysis concluded that “no new safety findings were identified” and the reports were consistent with the safety surveillance of the original vaccine course.

This data gathered during the large-scale rollout of vaccines for these children in the U.S. provides further evidence to support the safety findings of the clinical trials.

A death in a group of this size isn’t unusual

Almost 12,000 children took part in this trial over three years. With any group of this size, we would expect to see many adverse events happen over this time frame simply due to chance.

The risk of death varies significantly by age. For children aged one to four years old (covering the majority of ages included in the group), the death rate was 28 per 100,000 in the U.S. in 2022. Applying this rate to a group of 12,000 typical children, we would expect to see about three deaths over 12 months.

It is difficult to make a direct comparison to the children taking part in the KidCOVE trial, as they needed to meet certain health criteria. However, one death in this large group over the course of the trial appears to be well within expected normal bounds.

In conclusion, there was a report of a single death among 12,000 children taking part in a Moderna trial. While there is limited information about the death, Moderna’s report to the EU Clinical Trials Register indicated that it was unrelated to the vaccine. The trial finished in 2024 and only partial results have been fully published so far. However, a single death occurring in such a large group over more than a year is not unusual and would be expected from the typical death rate for children this age. Analysis of real-world vaccine safety data has not raised any concerns about mRNA vaccines for young children.

REFERENCES

• 1 – Chalkias et al. (2022) A Bivalent Omicron-Containing Booster Vaccine against Covid-19. New England Journal of Medicine.
• 2 – Creech et al. (2022) Evaluation of mRNA-1273 Covid-19 Vaccine in Children 6 to 11 Years of Age. New England Journal of Medicine.
• 3 – Anderson et al. (2022) Evaluation of mRNA-1273 Vaccine in Children 6 Months to 5 Years of Age. New England Journal of Medicine.
• 4 – Berthaud et al. (2024) Safety and Immunogenicity of an mRNA-1273 Booster in Children. Clinical Infectious Diseases.