Why are some vaccinations given subcutaneously (subcut) while others must be given intramuscularly (IM)?

In general, vaccines containing adjuvants (a component that enhances the antigenic response) are administered IM to avoid the risk of irritation, swelling, skin discoloration, inflammation, or granuloma formation if injected into subcutaneous tissue. This includes most of the inactivated vaccines, with a few exceptions (such as inactivated polio vaccine and pneumococcal polysaccharide vaccine, which may be given either subcut or IM). Vaccine efficacy may also be reduced if not given by the recommended route.