More than 300 cases of heart inflammation in young people have been recorded following Covid-19 vaccinations, according to Centers for Disease Control and Prevention Director Dr. Rochelle Walensky.

"Over 20 million adolescents and young adults [have been] vaccinated in the United States," Walensky said. "The cases are rare."

The CDC has advised that everyone aged 12 and above get vaccinated against Covid-19, and the vaccination drive is well underway: According to the CDC, over 7 million American teenagers and preteens (ages 12 to 17) have gotten at least one dose of a Covid-19 vaccine thus far.

Shortly after the Food and Drug Administration approved Pfizer's vaccine for use in children, however, authorities began receiving complaints of minor chest discomfort or other symptoms of potential heart inflammation (known as myocarditis) in a small percentage of recently vaccinated teenagers.

Myocarditis is an inflammation of the heart muscle, while pericarditis, which is also being studied, is an inflammation of the sac around the heart.

Long before the pandemic, thousands of cases of myocarditis were diagnosed each year in the U.S. and across the world, frequently as a result of the body's immunological response to diseases. SARS-CoV-2, the virus that causes Covid-19, as well as cold viruses, staph and strep, and HIV, can all cause it. Toxins and allergies are two more potential reasons.

The Advisory Committee on Immunization Practices, an independent advisory committee of the CDC, will meet next week to evaluate the cases. The meeting had been set for Friday, but it was postponed late Thursday when President Joe Biden signed a bill making Juneteenth a federal holiday. This year, because June 19 occurs on a Saturday, it will be observed Friday.

The panel will review the most recent research and safety data on myocarditis after vaccinations, but no changes to Covid-19 immunization guidelines are planned. Pfizer-vaccine BioNTech is the only one approved so far for children aged 12 to 17, however, young people 18 and older can obtain vaccinations from Pfizer, Moderna or Johnson & Johnson.

Although the CDC has said that evidence for a link is growing, it is uncertain if the vaccinations are the cause of the heart issue.