"The cases are rare," she said. "Over 20 million adolescents and young adults [have been] vaccinated in the United States."
Still, the cases of either myocarditis or pericarditis, which involve inflammation of the heart or the surrounding tissue, are higher than what would be expected for this age group. That's led the CDC to seek a meeting of a panel of independent experts called the Advisory Committee on Immunization Practices, who will gather Friday to review the cases.
The group will discuss the latest research and safety data on myocarditis following shots, but is not expected to make any changes to Covid-19 vaccination recommendations. The shot from Pfizer-BioNTech is the only one authorized so far for children ages 12 through 17, though young adults 18 and older can receive either Pfizer, Moderna or Johnson & Johnson's vaccines.
It remains unclear whether the vaccines are the cause of the heart problem, though the CDC has said that evidence for a link is growing stronger.
Dr. Paul Offit, a vaccine researcher at the Children's Hospital of Philadelphia, said Friday's meeting should result in a "clear understanding of the current risks for either children or young adults" from the vaccine.
- Watch TODAY All Day! Get the best news, information and inspiration from TODAY, all day long.
- Get TODAY's One Small Thing newsletter in your inbox each day.
During the briefing, Walensky said the CDC has asked "clinicians to be on the lookout for and report patients with symptoms of myocarditis or pericarditis following vaccination."
Symptoms can include fever and fatigue, as well as shortness of breath and chest pain. Most cases have not been serious.
"The vast majority have fully resolved with rest and supportive care," Walensky said.
Indeed, "people who have had this side effect tend to have had mild or minimal symptoms that resolve within several days," said Dr. Katie Passaretti, medical director for infection prevention at Atrium Health in Charlotte.
Friday's gathering of experts "is yet another demonstration of our ongoing efforts to keep safety central in everything we do," Walensky said. "Getting vaccinated is our way out of this pandemic."
This story first appeared on NBCNews.com.