evidence: In addition, public health experts are monitoring vaccine safety and side effect data for cases of rare heart-related complications, myocarditis or pericarditis, after receiving the Pfizer or Moderna vaccines. (Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the heart’s outer lining.) Research shows the benefits of COVID-19 vaccination outweigh the low risk of myocarditis after receiving the vaccine. Cases of myocarditis and/or pericarditis have been more common in adolescent boys and young men. A 2021 study shows that most people under age 21 who developed myocarditis thought to be linked to COVID-19 vaccination had mild symptoms that improved quickly. [...] As part of the normal immune reaction that occurs after COVID-19 vaccination, lymph node swelling can occur in the underarm near where you received your shot. That swelling may show up on your mammogram and can raise a false alarm.