The Centers for Disease Control and Prevention (CDC) is investigating reports of myocarditis, inflammation of the heart muscle, in adolescents and young adults who were injected with an mRNA COVID-19 vaccine.

In a report posted on its website last month, the CDC said the cases occurred within four days of being injected with the second vaccine dose. Males also reported more cases than females. But health experts have yet to determine whether myocarditis and the experimental COVID-19 vaccines are related and how, if so.

Biotechnology company Moderna said Tuesday, May 25, that its COVID-19 vaccine was “100 percent effective” in a study of adolescents ages 12 to 17 years. That would make Moderna the second vaccine manufacturer behind Pfizer-BioNTech to demonstrate the high efficacy of its COVID-19 vaccine in younger age groups.

In light of this study, Moderna said it plans to ask the Food and Drug Administration to expand the emergency use of its COVID-19 vaccine for teenagers beginning June. If approved, Moderna would likely expand the number of vaccine doses available to middle and high school students ahead of the next school year.

Despite the reports, the CDC continues to urge adolescents aged 12 years and older to get vaccinated.

Myocarditis: symptoms and risk factors

Myocarditis is a rare condition that can occur at any age. It is also typically caused by a viral illness, according to Michael Carr, a cardiologist at the Ann & Robert H. Lurie Children’s Hospital of Chicago. He said they typically see approximately 10–12 cases of myocarditis that require hospital admission each year.

In some cases, myocarditis is also caused by the body’s immune reaction to the initial heart damage, according to the National Organization for Rare Disorders, a nonprofit patient advocacy organization.

Symptoms of myocarditis include chest pain, an abnormal heart rhythm, shortness of breath and fatigue. There is no specific treatment for the condition. Physicians usually advise resting and taking anti-inflammatory drugs to decrease the inflammation and alleviate the symptoms, added Carr.

In a majority of cases, symptoms of myocarditis are preceded a few days or weeks by a flu-like illness. Though usually mild, myocarditis can lead to life-threatening events, including heart failure, heart attack, sudden cardiac death and stroke.

Risk factors for myocarditis include chemical exposure, radiation, infection, use of illegal or prescription drugs and underlying diseases, especially autoimmune disorders.

Israel reports link between myocarditis and Pfizer vaccine

In a report submitted Tuesday, June 1, to the Israeli Ministry of Health, researchers said between one in 3,000 and one in 6,000 men ages 16 to 24 years who were vaccinated with the Pfizer-BioNTech vaccine developed myocarditis. But they also found that most of the cases were mild and were resolved within a few weeks.

Nonetheless, the report suggests that the Pfizer-BioNTech vaccine puts young men at an elevated risk of developing myocarditis.

Israeli health officials first noticed the issue in April, when they reported over 60 cases of myocarditis in young men who received the second dose of the Pfizer-BioNTech vaccine just a few days prior to developing the condition. 

It was also around that time that the U.S. Department of Defense started tracking 14 cases of myocarditis. Officials at the E.U.’s European Medicines Agency also said they received 107 reports of myocarditis following the Pfizer-BioNTech vaccine last month.

The Israeli researchers’ findings come as Israel and several European countries are debating whether younger adults and adolescents should be vaccinated against COVID-19. Israel has been vaccinating adolescents aged 16 years and older since late January. Other countries, including the U.S. and Canada, have been vaccinating adolescents aged 12 years and older since mid-May.

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