Which of the following routes is NOT commonly used for vaccination?

The route that is not commonly used for vaccination is intravenous. Vaccines are generally administered via methods that promote an immune response at the site of injection or via mucosal surfaces. Intranasal, intramuscular, and subcutaneous routes are well-established for vaccines and are selected based on the immune response desired and the characteristics of the vaccine.

Intranasal vaccination takes advantage of the mucosal immune system and is effective for certain vaccines, including the live attenuated influenza vaccine. Intramuscular vaccination is one of the most common methods, allowing for the delivery of vaccines into muscle tissues, which helps with a robust immune response. Subcutaneous vaccination is also frequently used, particularly for certain live vaccines that require slower absorption.

Intravenous administration, on the other hand, is not a typical route for vaccines since it can lead to systemic effects that are not desirable when the goal is to elicit a localized immune response. Most vaccines are specifically formulated to be administered in ways that minimize risks and optimize immune outcomes, making intravenous administration unsuitable as a standard route for vaccination.