Check out this paper published in JAMA written by a bunch of CDC authors including my favorite CDC misinformer, Dr. John Su.

It's super popular. Over 500,000 views.

Here's the key paragraph:

Furthermore, as a passive system, VAERS data are subject to reporting biases in that both underreporting and overreporting are possible.38 Given the high verification rate of reports of myocarditis to VAERS after mRNA-based COVID-19 vaccination, underreporting is more likely. Therefore, the actual rates of myocarditis per million doses of vaccine are likely higher than estimated.