Researchers probe link between Covid-19 vaccines and myocarditis
Researchers are uncertain why messenger RNA vaccines, one from Pfizer Inc. and its partner BioNTech SE and the other from Moderna Inc., are likely to be the cause of inflammatory heart disease, myocarditis, and pericarditis in a small number of cases.
Some theories focus on the type of spike protein that a person makes in response to mRNA vaccines. The mRNA itself or other components of the vaccines, the researchers said, could also trigger certain inflammatory responses in some people.
A new theory under review: inappropriate injections of the vaccine directly into a vein, which sends the vaccine to the heart muscle.
To find answers, some doctors and scientists perform tests in laboratory dishes and examine heart tissue samples from people who have developed myocarditis or pericarditis after being vaccinated.
Myocarditis describes the inflammation of the heart muscle, while pericarditis refers to the inflammation of the sac surrounding the muscle.
Covid-19 itself can cause both conditions. They have also been reported in a smaller number of people who received an mRNA vaccine, most often in men under 30 and adolescents.
About 877 confirmed cases of myocarditis in vaccinated people under the age of 30 have been reported in the United States, out of 86 million doses of mRNA vaccine administered, according to the Centers for Disease Control and Prevention.
The risk is highest within seven days of the second dose of Pfizer-BioNTech and Moderna vaccines, according to the Food and Drug Administration. Most cases of myocarditis in people who have been vaccinated are relatively mild and patients recover on their own or with minimal treatment, doctors say.
CDC recommends anyone 5 years of age and older get vaccinated, saying the benefits of preventing Covid-19 disease, hospitalizations and death far outweigh the risk of myocarditis, even in younger men .
Researchers have tried to understand the link between cases and vaccination, as health officials expand the vaccination campaign by recommending boosters and expanding the use of the Pfizer-BioNTech vaccine to young children.
The FDA, however, suspended the authorization for the use of the Moderna vaccine in adolescents as it investigated the risk.
Some theories about the vaccine’s link to myocarditis focus on the spike protein on the surface of the coronavirus.
The spike protein helps the virus enter human cells to replicate. MRNA vaccines are designed to cause the body to make a certain version of the spike protein, which then triggers an immune response.
The immune response includes neutralizing antibodies that target the spike protein and thereby block the ability of the virus to enter cells and replicate. The immune response can protect a person from Covid-19 or reduce its severity if someone is exposed to the virus.
Still, there may be similarities between the spike protein and proteins found in heart muscle, causing the body’s immune defenses to mobilize against the heart, according to Biykem Bozkurt, professor of medicine specializing in cardiology at Baylor College of Medicine. in Houston.
Antibodies to the spike protein may have the unintended effect of acting against heart proteins, said Dr Bozkurt, who co-authored a review of vaccine-associated myocarditis in the journal Circulation in July.
This theory of “molecular mimicry” has not been fully tested in people who have been vaccinated and does not explain why myocarditis only occurs in some people, she said.
Some of the mRNAs in vaccines can also be taken up by heart cells called cardiomyocytes, said Jay Schneider, a cardiovascular medicine consultant at the Mayo Clinic campus in Jacksonville, Fla.
These cells can then produce the coronavirus spike protein, which could elicit an antibody response against them, Dr Schneider said. He said he performed lab tests and found that heart cells had adopted the Moderna vaccine and then expressed the spike protein.
Dr Schneider has yet to publish the results of these experiments in a peer-reviewed journal and said they should be interpreted with caution.
Some doctors have speculated that improper vaccine injections could help.
The injections are meant to be injected into the muscle of the shoulder, also known as the deltoid muscle. If the injection accidentally reaches a vein, it could lead to some of the vaccine being given to the heart through the blood vessels.
Researchers in Hong Kong have found that intravenous injection of Pfizer-BioNTech vaccine into mice induces both myocarditis and pericarditis in animals, according to results published online in August by the journal Clinical Infectious Diseases.
The fact that myocarditis seems to occur more in young men after vaccination than in other age and gender groups suggests a link with the hormone testosterone, which is generally at high levels in young men, according to the researchers. researchers.
Testosterone can increase an inflammatory immune response, Dr Bozkurt said, leading to myocarditis in some adolescents and young men.
Some studies have suggested that Moderna’s vaccine carries a higher risk of myocarditis than that of Pfizer. If this difference is confirmed, it could be linked to differences in dose levels, said Moderna CEO Stéphane Bancel. Moderna’s vaccine contains more mRNA per dose than that of Pfizer.
Some vaccine specialists have cited the dose difference as one of the reasons the Moderna vaccine’s effectiveness against Covid-19 appears to outlast Pfizer’s.
Mr Bancel said that the benefit-risk balance of Moderna’s vaccine is still favorable for young men.
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