Oral vaccines are gaining more attention due to their ease of administration, lower invasiveness, generally greater safety, and lower cost than injectable vaccines. This review introduces certified oral vaccines for adenovirus, recombinant protein-based, and transgenic plant-based oral vaccines, and their mechanisms for inducing an immune response. Procedures for regulatory approval and clinical trials of injectable and oral vaccines are also covered. Challenges such as instability and reduced efficacy in low-income countries associated with oral vaccines are discussed, as well as recent developments, such as Bacillus-subtilis-based and nanoparticle-based delivery systems that have the potential to improve the effectiveness of oral vaccines.
