Different tissues are protected by different types of antibodies. A vaccine injected subcutaneously will mostly induce an IgG response, a type of antibody prese

Different tissues are protected by different types of antibodies. A vaccine injected subcutaneously will mostly induce an IgG response, a type of antibody present in blood and extracellular fluid. This could still possibly allow a virus to infect mucous membranes, which rely on a different type of antibody, sIgA[0], to protect them. If this is the case, you could be protected from having a serious systemic infection, but could still suffer a mild upper respiratory infection, and be infectious until the immune system mounted defense in the mucous membranes. Some flu vaccines come in the form of a nasal spray, which does induce an IgA response. I don't know why COVID vaccines are administered parenterally instead, I'm sure there are good technical reasons. Given the virus's possible neurological involvement, maybe it's a bad idea to deposit it so close to the olfactory bulb. 0: https://en.wikipedia.org/wiki/Immunoglobulin_A#Secretory_IgA https://en.wikipedia.org/wiki/Immunoglobulin_A#Secretory_IgA