International Journal of Molecular Veterinary Research, 2024, Vol.14, No.5, 202-210 http://animalscipublisher.com/index.php/ijmvr 203 2 Overview of Poultry Vaccines 2.1 Types of vaccines used in poultry Poultry vaccines are essential tools in the prevention and control of infectious diseases in poultry farms. They play a critical role in maintaining the health of poultry, ensuring food security, and minimizing economic losses due to disease outbreaks (Zhou and Lin, 2024). Poultry vaccines can be broadly categorized into live attenuated vaccines, inactivated vaccines, and recombinant vaccines (Table 1) (Wang et al., 2024). Live attenuated vaccines are derived from pathogens that have been weakened so they do not cause disease but still elicit an immune response. Inactivated vaccines, on the other hand, are made from pathogens that have been killed and are safe but may require adjuvants to enhance their efficacy (Bublot, 2023). Recombinant vaccines, including those using virus-like particles (VLPs) and viral vectors, represent a more recent advancement. These vaccines can target multiple diseases simultaneously and are designed to induce a strong immune response even in the presence of maternal antibodies (Romanutti et al., 2020; Raji et al., 2024). Table 1 Characteristic of conventional vaccines and vector vaccines (Adopted from Wang et al., 2024) Inactivated vaccines Attenuated vaccines Vector vaccines Safety The best security Some have potential risk of virulence reversion Some have potential risk of virulence reversion Effect of the maternal antibody Depending on the level of antibodies Depending on the level of antibodies Some can effectively avoid the interference of maternal antibodies, such as HVT-based vaccine Duration of immunity Can only induce a short period of immune protection and requires multiple immunizations Can induce a long period of immune protection Can induce a long period of immune protection Immune response The onset of protection was short but mainly induced humoral immunity. Some produced only a local response The onset of protection was long, but both humoral and cellular immunity could be induced The onset of protection was long, but both humoral and cellular immunity could be induced Cost Less expensive More expensive Variable 2.2 Common diseases targeted by vaccination Vaccination in poultry primarily targets viral diseases such as Newcastle disease, avian influenza, Marek’s disease, and infectious bursal disease. These diseases are significant due to their impact on poultry health and the potential for economic losses. Recombinant viral vector vaccines have been developed to combat these diseases effectively, offering protection against multiple pathogens with a single vaccine (Hein et al., 2021; Ravikumar et al., 2022). Additionally, infectious laryngotracheitis (ILT) is another disease for which both live attenuated and recombinant vaccines are available, with ongoing research to improve their efficacy (García, 2017). 2.3 Vaccine development and approval process The development of poultry vaccines involves several stages, including the identification of protective antigens, vaccine formulation, and testing for safety and efficacy. Recombinant DNA technology has facilitated the creation of novel vaccines by allowing the expression of specific antigens in viral vectors, such as fowlpox virus and turkey herpesvirus (Wang et al., 2024). The approval process for these vaccines requires rigorous testing to ensure they are safe, effective, and capable of inducing a protective immune response. This includes evaluating their ability to reduce virus shedding and prevent clinical disease in vaccinated birds (Nielsen et al., 2023). The development of vaccines also considers factors such as cost-effectiveness, ease of administration, and the ability to differentiate vaccinated from infected birds (DIVA strategy) (Suarez and Pantin-Jackwood, 2017). 3 Field Trial Methodologies for Vaccine Efficacy 3.1 Experimental design and farm selection Field trials for assessing vaccine efficacy in poultry farms involve careful experimental design and farm selection to ensure reliable results. For instance, in a study evaluating the Newcastle disease I2 vaccine, twenty households
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