I'm betting the vaccine triggered the cytokine storm, rather than a virus itself which the most common triggering event. It's such a rare disease that by the time it's diagnosed, it's too late to intervene with steroids and chemo drugs to stop the runaway immune system.
https://www.dana-farber.org/childhood-hemophagocytic-lymphoh...
I'm betting the vaccine triggered the cytokine storm, rather than a virus itself which the most common triggering event. It's such a rare disease that by the time it's diagnosed, it's too late to intervene with steroids and chemo drugs to stop the runaway immune system.