Journal of Vaccine Research, 2025, Vol.15, No.1, 34-44 http://medscipublisher.com/index.php/jvr 39 4.3 Zika and leishmaniasis The development of the Zika vaccine mainly employs DNA, mRNA and viral vectors and other means. In animal experiments and early human tests, some candidate vaccines have already demonstrated effects that can stimulate human immunity (Huang et al., 2023; Rothen et al., 2024). However, as it is difficult to predict when the Zika virus will break out, there is no reliable way to assess the effectiveness of the vaccine at present. Moreover, people are also concerned that the vaccine may have cross-reactions with dengue fever, posing a risk of ADE, which has led to a slow progress in clinical trials (Poland et al., 2018; Mourosi et al., 2022). The development of leishmaniasis vaccines has been progressing unsmoothly. Most of the candidate vaccines have only been effective in animal experiments and few have entered the human trial stage. Although patients in areas with a high incidence of these diseases suffer greatly, in resource-scarce regions, the difficulty in making profits from vaccine research and development and the low capital investment further hinder the research and development process (Poland et al., 2018; Al-Osaimi et al., 2024). These difficulties indicate that to promote the research and development of vaccines against such neglected vector-borne diseases, it is necessary to increase financial investment, innovate research and development methods, and carry out global cooperation. 5 Major Challenges in Vaccine Development 5.1 Scientific and technological obstacles Many pathogens transmitted through media have complex biological characteristics and can mutate rapidly, which poses a significant scientific challenge to vaccine development. These pathogens have numerous genetic variations and significant differences in surface antigens, making it difficult to identify stable and widely protective vaccine development targets. Moreover, our understanding of the principles by which the human body generates immunity against these pathogens is not deep enough, which makes it even more difficult to design effective vaccines (Kennedy et al., 2020; Heaton, 2020; Hamisi et al., 2024). The genetic differences between the human body and pathogens, coupled with the mutual influence of multiple immune and microbial defense mechanisms, have made the standards for determining the protective effect of vaccines ambiguous. Due to insufficient understanding, obstacles will be encountered when predicting the efficacy of vaccines and designing vaccines that can trigger ideal immune responses, especially for those diseases with poor natural immune responses or unclear immune principles (Bekeredjian-Ding, 2020; Kennedy et al., 2020; Hamisi et al., 2024). 5.2 Clinical and regulatory barriers Because the outbreak time of vector-borne diseases is uncertain and the number of cases is unstable, it is very difficult to design and carry out clinical trials of related vaccines. Disease outbreaks may be irregular or occur only in local areas, making it difficult to recruit enough trial participants and to conduct trials on the target population at the right time. Facing such uncertainties, it is necessary to adopt innovative and flexible experimental design methods, and also rely on mathematical and statistical models to optimize the research plan and evaluation results (Bekeredjian-Ding, 2020; Madewell et al., 2021; Bailon et al., 2022). The vaccine approval process is both strict and slow, and sufficient safety and effectiveness proof must be presented. This depends on large-scale and standardized tests. Moreover, conducting research in areas with poor conditions is inherently troublesome, and all these will delay the development and application of vaccines. Furthermore, disease prevention methods are constantly changing, and there are ethical issues, such as using existing prevention and control methods in the control group. All these make the approval process more difficult (Pitsutithum and Marovich, 2020; Madewell et al., 2021; Weerarathna et al., 2024). 5.3 Economic and social issues The lack of funds has greatly slowed down the research and development of vaccines for many insect-borne diseases, especially those that are prevalent in poor areas. Because the profit margin is not high and not much can be made, few people are willing to invest in these vaccines, especially those mainly for the poor. As a result, the development of new vaccines was slow and they were ineffective against these often underestimated diseases (Voss et al., 2018; Heaton, 2020).
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