There is mounting evidence that the mRNA COVID-19 vaccines by Pfizer and Moderna could cause heart inflammation, especially in young men. One doctor said heart inflammation can be caused by certain vaccines — the smallpox vaccine was known to cause it. Now, the Centers for Disease Control and Prevention is investigating whether the mRNA COVID-19 shots do, as well."They haven't definitively made a causal link. What they've seen now is an association that seems to be getting a little bit stronger evidence," said Dr. Amesh Adalja, of the Johns Hopkins Bloomberg School of Public Health.Stronger evidence that the mRNA COVID-19 vaccines, from Pfizer and Moderna, could be causing myocarditis, inflammation of the heart muscle or pericarditis, inflammation of the lining of the heart, especially in males aged 16-24."Probably due to the inflammation that occurs when you get the vaccine the immune reaction may not only generate antibodies and T-cells against COVID-19 but may also generate some reaction against parts of the heart muscle that's likely what the hypothesis is and this is something that needs to be worked out," Adalja said.The CDC is looking into 475 cases in people under the age of 30 since April. As of May 31, 270 were hospitalized and released, 15 were still hospitalized and three patients were in the ICU. Not all have yet been linked to the vaccines, and the vast majority are considered mild cases."Most cases of myo (myocarditis) or pericarditis are going to be hospitalized because we hospitalize people who have chest pain, we hospitalize people who have evidence of heart damage on an EKG or bloodwork, so that's going to kind of make hospitalization not the way we define severity here," Adalja said.Adalja said anyone who is concerned and is over 18 can get the Johnson & Johnson COVID-19 shot instead, but it's a very rare possible side effect and he still recommends everyone, even young men, get vaccinated against COVID-19."There are some people that are advocating and I think with good reason about getting first doses in that group, then maybe delaying the second dose because there's a hypothesis that maybe that second dose adds inflammation onto the first dose and if you space them, you'll still get the benefit of the vaccine, but the inflammation will have dampened and you'll be less likely to have those problems," Adalja said.The CDC is holding an emergency meeting to discuss this issue on Friday. Doctors expect to get more clear guidance after that.