[shmooper]

COVID's most dangerous symptom is a condition called cytokine storm[1]. Cytokines are molecules used by the immune system as a means for communication, such as calling for help, cell destruction and more. COVID creates a "storm" of signals, driving the immune system crazy. As a part of the immune system reaction to those signals, inflammation can occur (as a way to isolate and fight a specific infected zone). During this attack, the immune system sends out cells from the blood and into the organs[2], making the blood vessels more permeable. I'm not certain but I believe that's the cardiovascular symptoms you're referring to.

1. The COVID-19 Cytokine Storm; What We Know So Far https://www.frontiersin.org/articles/10.3389/fimmu.2020.0144... 2. Leukocyte Migration into Inflamed Tissues https://www.sciencedirect.com/science/article/pii/S107476131...