The mRNA COVID-19 Vaccines may be inducing a short-lived temporary immunosuppression allowing cytomegalovirus (CMV) to become reactivated in some people in very rare cases. This reactivation of the cytomegalovirus can in rare circumstances cause myocarditis and Guillain-Barré syndrome and a host of other ailments. The cytomegalovirus is highly ubiquitous in nature and common in people of all ages and is the cause of chickenpox and mononucleosis in adolescents. After infection, CMV remains dormant in the body of most humans throughout their lives, but can become reactivated during immune suppression. A decreasing CMV susceptibility with men older than 45 may be the reason why rare cases of myocarditis is happening in younger people who have taken the mRNA vaccine. CMV susceptibility increases between the ages of 16-45. Treatments, medications, and even vaccines can temporarily suppress the immune system and cause CMV reactivation. This is however very rare, but should be looked into as a possible cause of rare instances of myocarditis and Guillain-Barré in those who have taken the COVID-19 mRNA vaccine.