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> In children who have not already had dengue, the dengue vaccine increases the risk of hospitalization and severe illness if the child gets dengue after vaccination.

The mechanism is most likely that wild-type dengue has optimized itself a bit to infect better when covered with vaccinal antibodies (antibody-dependent enhancement)

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Dengue is one of the few diseases that it explicitly targets immune cells. It hits Langerhans cells, which are skin-resident monocytes (a type of immune cells), spreads through the lymphatic system (part of the immune system), and then to white blood cells.

While dengue is certainly not unique (e.g. HIV also targets the immune system), but it is in the extreme minority of diseases. For the vast majority of diseases, this kind of response to vaccines or prior infections is simply not a thing.

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