Researchers at Stanford University have unveiled insights into how mRNA Covid vaccines might impact heart cells, shedding light on a rare but concerning side effect known as myocarditis. This condition, which involves the inflammation of the heart muscle, has been a topic of concern among medical experts for some time.

The incidence of myocarditis remains extremely low, occurring in approximately one out of every 125,000 individuals under the age of 65 who were vaccinated between 2023 and 2024. However, the risk appears slightly higher in men and boys under 25, with statistics indicating a rate of one in 250, or 475 per 125,000.

In their study, the Stanford team examined the blood samples of vaccine recipients, comparing those who developed myocarditis with those who did not. Their findings revealed that individuals who experienced this rare side effect had heightened levels of two proteins associated with the immune system present in their bloodstream.

These discoveries could pave the way for a deeper understanding of the biological mechanisms behind this rare reaction, potentially guiding future research and vaccine development to mitigate such risks. While the occurrence of myocarditis post-vaccination is uncommon, this study provides valuable insights that can enhance both medical knowledge and public health strategies moving forward.

They found that people who suffered from the rare side-effect had elevated levels of two immune system-linked proteins in their blood.

Subsequent experimentation in mice then showed that these proteins prompted neutrophils (a type of immune cell that is the first responder to damage) to infiltrate the heart muscle, causing damage to the tissue via inflammation.

Doctors say the complication is normally mild and eases on its own without treatment. In severe cases, however, it can cause hospitalization or death, although officials say no fatalities from the condition stemming from the Covid vaccine have been recorded in the US to date.

The CDC warns that an infection with the Covid virus can also cause myocarditis, suggesting the benefits of vaccination may still outweigh the risks. About 21 per 100,000 people suffer from the complication after a Covid infection, according to a 2022 study from Italy.

Scientists at Stanford University have revealed how the mRNA Covid vaccines can cause damage to heart cells (stock image of a man being vaccinated)

The paper comes amid renewed focus on the risks of Covid vaccines, after the director of the FDA’s vaccine division, Dr Vinay Prasad, told staff in a memo late last month that there had been ‘no fewer than 10’ child deaths ‘related’ to the Covid vaccine.

His comments were based on data from the agency’s VAERS, or the Vaccine Adverse Event Reporting System, a voluntary database for reporting vaccine side-effects. Anyone can submit a report to the system, and the reports do not prove that a shot caused a complication.

The FDA has said it is now conducting a thorough investigation into reports of deaths in both children and adults that are thought to be linked to the Covid vaccine.

About 270 million people in the US, or 81 percent of the population, have received at least one dose of the mRNA Covid vaccine to date.

Dr Joseph Wu, a cardiologist at Stanford University who led the research, said in a press release: ‘The mRNA vaccines have done a tremendous job mitigating the Covid pandemic.

‘Without these vaccines, more people would have gotten sick, more people would have had severe effects and more people would have died.’ 

In the study, published yesterday in Science Translational Medicine, scientists analyzed in the lab the blood of people who received the Covid vaccine.

In those who also developed myocarditis, the researchers found higher levels of two proteins, CXCL10 and IFN-gamma.

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 vaccine, according to a 2022 study. Risk was highest after the second dose

Rory Nairn (pictured here), a 26-year-old from New Zealand, died from myocarditis 12 days after receiving his first dose of the Pfizer Covid vaccine in 2021

These are cytokines, proteins released by immune cells that can prompt inflammation.

In a lab test, researchers then placed macrophages, a type of white blood cell, into petri dishes filled with the Covid mRNA vaccine, and found that the cells released CXCL10.

They then placed T-cells, a type of immune cell, into those same petri dishes and found they released high amounts of IFN-gamma.

When the T-cells were placed into petri dishes that only contained the vaccine, however, the researchers found they only released standard amounts of IFN-gamma.

This showed that, in some cases, macrophages may be triggered by Covid vaccines and then cause other immune cells, T-cells, to also be activated. 

In a further test, the researchers then inoculated young adult male mice with the Pfizer mRNA Covid vaccine and, after a few days, examined the animals’ heart muscle.

It showed that their hearts had been infiltrated by neutrophils, which researchers said showed how inoculations may cause heart damage.

Rhian Griffiths, from the UK, was otherwise fit and healthy when she suffered a cardiac arrest in 2023 at age 32. After her death, she was diagnosed with myocarditis. Her family suggested her ‘unknowingly catching Covid had this effect on her heart’

Anti-vaxxers have been urging for Covid shots to be withdrawn from the market for years over heart damage fears, arguing that, in some groups, the risk is greater than if they had been infected instead.

Dr Wu added in the press release: ‘Other vaccines can cause myocarditis and inflammatory problems, but the symptoms tend to be more diffuse.

‘Plus, mRNA-based Covid vaccines’ risks have received intense public scrutiny and media coverage.’ 

He added: ‘If you get chest pains from a Covid vaccine, you go to the hospital to get checked out, and if the serum is positive, then you get diagnosed with myocarditis.

‘If you get achy muscles or joints from a flu vaccine, you just blow it off.’

Earlier this month Dr Robert Redfield, who headed the CDC during Trump’s first term, said he did not support the continued use of mRNA Covid vaccines.

In September, the CDC’s vaccine panel, the Advisory Committee on Immunization Practices (ACIP), voted to remove the recommendation that everyone aged six months and older should get the Covid vaccine, and instead told Americans to get the vaccine based on ‘individual decision-making’.

Health and Human Services Secretary Robert F Kennedy Junior, who selected the ACIP panel, has previously criticized the Covid vaccine rollout.