Since many of the most virulent pathogens infect through a mucosal (respiratory, gastrointestinal, or genitourinary tract) surface, vaccination at these sites is often the strategy to generate local prophylactic response(s). A major limitation of mucosal vaccination is the challenge of delivering non-pathogenic immunogens into the epithelial cells and/or the underlying lamina propria in a manner that emulates an actual infection by that pathogen. As one might expect, there are many physical, physiological, and biological hurdles in place at these mucosal surfaces to maintain homeostasis and make it difficult for materials the size of viruses to enter. Pathogens, however, have developed elaborate mechanisms to overcome these hurdles through their infective processes. Thus, there are severe challenges for safe and efficient immunogen delivery at mucosal surface to achieve a consistent immunization. An added issue is that mucosal immunity appears to be more dynamic than systemic immunity, likely in response to the ever-changing environmental challenges being confronted, with the likelihood of vaccination boosts being needed in the timeframe of months rather than years to maintain truly prophylactic protection.

While mucosal vaccination can also generate a corrective vaccination outcome, the complex delivery challenges associated with mucosal vaccination can be eliminated by parenteral injection, commonly achieved via the IM, SC, or ID routes. Intravenous (IV) administration can technically be used for parenteral vaccine administration, but this route is not generally used since vaccines are commonly formulated as a depot to incite a more robust and durable response, and such depots are not amenable for administration by the IV route [2]. There are several anatomical distinctions to be considered, however, between IM, SC, and ID vaccine injection routes. The ID injection site is a dense connective tissue bed that contains hair follicles, blood vessels, lymphatic vessels, and sweat glands with limited potential for expansion. By comparison, injections into the SC site enter into an expandable space composed of a loose connective tissue structure housing adipocytes. The IM injection site is also an expandable environment able to accept an injected material as it is dispersed between muscle fascicles. SC injections for vaccination are commonly from 1–1.5 mL in volume [3], IM injection are 2–5 mL [4], but 50 µL or less for ID injections [5]. Transition from an SC injection to ID typically involves reducing the injected dose volume to 10–20% of the full dose delivered either SC or IM while remaining safe and efficacious. Multiple studies comparing the full dose IM to reduced dose ID administrations of inactivated poliovirus have shown sufficiently promising results to warrant adoption of this change for both routine and supplementary vaccinations as a dose-sparing strategy [6].

ID vaccinations were probably the first intentional immunization strategy, being achieved by applying inactivated whole pathogens to skin sites damaged by physical scoring [7]. This approach allowed the host’s immune system to establish an immunity to the pathogen based upon an individual’s unique immune system repertoire. With the advent of needles and syringes, it became possible to deliver an exact quantity of a vaccine material in a reproducible manner, leading to SC or IM injections becoming a standard immunization strategy. With such standardized injections came the concept of augmenting the immune response by incorporating adjuvants as part of an injected depot [8]. The outermost layer of human skin is known as the epidermis with a thickness of 0.05–0.2 mm; beneath the epidermis is the dermis which is a network of collagen fibers only 1.5–3 mm thick with the SC space below that being quite viable in thickness. IM injections are administered at a 90º angle to the skin surface with a needle that is sufficiently long to reach a striated muscle after penetrating the epidermis, dermis, and SC tissues, while SC injections are given at a 45º angle. ID injections, however, are given and at a 10–15º angle. This very low angle, known as the Mantoux technique, can be awkward to get a needle correctly placed prior to administration [9].