Well, considering that current treatments for these diseases involve a broad based suppression of the immune system which lead to substantial increases risk from routine infection, they would have to be pretty bad to do worse than the current standard of care.
For example, the near state-of-the-art in immune suppression is the IL-17 inhibitor. IL-17 itself is a crucial signaller for Cytokine production. And it signals a broad array of Citokines. Citokines are essential in fighting most forms of infection. So, to have side effects as bad as a modern immune suppressant, an "inverse vaccine" would have to have a generalized effect as bad as global Citokine suppression. That would be an extraordinary leap for a drug which is meant to tell T cells to ignore one specific molecule.
For example, the near state-of-the-art in immune suppression is the IL-17 inhibitor. IL-17 itself is a crucial signaller for Cytokine production. And it signals a broad array of Citokines. Citokines are essential in fighting most forms of infection. So, to have side effects as bad as a modern immune suppressant, an "inverse vaccine" would have to have a generalized effect as bad as global Citokine suppression. That would be an extraordinary leap for a drug which is meant to tell T cells to ignore one specific molecule.