Most infectious agents enter the body on the surfaces of mucous membranes, which are equipped with a mucosal immune system that serves as the first line of defence against pathogens. Protective mucosal immune responses can be triggered by mucosal immunisation, which is why the development of mucosal vaccines is increasingly being promoted as an alternative to the currently prevalent intramuscular vaccines. The mucosal immune system is part of the innate immune system, but it also forms a link with the acquired immune system, so mucosal vaccines could achieve dual protection. Despite the many advantages, there are challenges in developing safe, stable and appropriately formulated mucosal vaccines. Different options for delivery systems and dosage forms are being explored to optimise their performance. The nasal route is the most commonly used route for the administration of mucosal vaccines, and nasal vaccines are therefore currently the most studied. There are two effective licensed nasal influenza virus vaccines on the market, FluMist® and Nasovac®. Nasal vaccines also have great potential in the fight against SARS-CoV-2, but most of these vaccines are still in the early stages of clinical trials.
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