Thousands of American teenagers may have suffered heart inflammation after getting a Covid jab, a study suggests.
Researchers found up to one in 7,000 boys aged 12 to 15 years old developed myocarditis after receiving the Pfizer vaccine.
The condition — which is mild for most but can cause a recurrent heart palpitation in rare cases — was most common after the second dose.
Covid is also mild for most teenagers with only 1,745 Americans under-18 dying from the disease compared to more than 800,000 over-50s.
Experts said their paper was not proof people should stop being vaccinated, adding the benefits still ‘greatly outweigh’ the risks.
Covid itself is known to cause myocarditis, with some studies known it is more common from infections than after an inoculation.
Vaccinating children has been hugely contentious during the pandemic because of the smaller risks Covid poses to them compared to older adults.
The above graph shows the risk of suffering myocarditis by doses, based on people under 40 years old and up to seven days after getting their jab. Risk was highest after the second dose
Researchers at the healthcare provider Kaiser Permanente reviewed 340 cases of myocarditis and pericarditis in hospitals across eight US states.
Myocarditis is when the heart muscle becomes inflamed, while pericarditis is when the lining of the heart becomes inflamed.
Overall, across all under-40s the rate of the side-effect was one in 30,000 after a second dose.
It was one in 200,000 after a first jab, and one in 50,000 after a booster dose.
Experts did not attempt to explain the difference between doses, although it may have something to do with the immune system responding for a second time.
Myocarditis is triggered when the body’s immune system causes inflammation — often because it is battling an infection such as from Covid.
The findings were published today in the Annals of Internal Medicine.
For the paper, they monitored 6.9million Covid vaccine recipients for whether they visited an ER ward with myocarditis.
This included symptoms such as a fluttering heart or being short of breath within a week of being vaccinated.
All participants got their jabs between December 14, 2020 and May 31, 2022, across eight US states including California, Colorado and Georgia.
A total of 119 myocarditis cases were confirmed out of 3.4million people who got their second dose.
For those who got their first jab, there were 18 out of 3.5million administered.
And among the 1.8million participants in the study, 77 suffered the condition.
Cases were included if they were detected within seven days of vaccination, and could be independently verified by researchers.
Children or teenage boys aged 12 to 15 years old were most likely to suffer myocarditis after a second dose.
On the other hand, women aged 18 to 29 years old rarely experienced the side effect.
The study did not look at the rates between Pfizer and Moderna vaccines, although Moderna recipients are thought to have a slightly higher risk.
It also did not include a control group, a population that was monitored for myocarditis after not receiving a Covid vaccine.
Dr Kristin Goddard, from Kaiser Permanente who led the research and others, said the figures did not show that people should stop getting Covid vaccines.
‘Our findings can inform risk-benefit analysis, which thus far have consistently found the benefits of mRNA vaccination greatly outweight the risks.
‘Continued communication with patients and providers about risk for myocarditis and pericarditis after Covid vaccination… is warranted.’
Data was from the Vaccine Safety Datalink System, which involves hospitals and ER rooms in eight states.
Some warned the figures may represent under-reporting, because some patients do not notice they have myocarditis or pericarditis.
The Centers for Disease Control and Prevention (CDC) says that myocarditis is rarely reported, but most likely in adolescents and young males.
Analysis carried out for the agency and published in April this year found the rates were up to one in 600 for boys aged 12 to 15 years after the second dose.
But it was much less common among women in the 18 to 29 age group, with very few cases identified.
This analysis did not include boosters.
WHAT IS MYOCARDITIS?
Myocarditis is an uncommon disorder. Most of the time, it is caused by an infection that reaches the heart
Myocarditis is heart inflammation caused by a viral infection, such as COVID-19.
In severe cases, the inflammation can weaken the heart, cause an abnormal heartbeat, or even lead to death.
Symptoms typically include chest pain or shortness of breath.
Patients can treat the condition with medication aimed at regulating heartbeat and improving heart function. Some rare cases have required patients to have a device implanted in their heart to regulate heartbeat.
Myocarditis is a mild, temporary condition in the vast majority of cases, experts say.
Heart inflammation is uncommon in pro athletes who’ve had a mild COVID-19 infection and most don’t need to be sidelined, according to a 2021 study conducted by major professional sports leagues.
This illustration shows normal heart muscle compared to inflamed heart muscle due