Journal of Vaccine Research, 2025, Vol.15, No.1, 1-9 http://medscipublisher.com/index.php/jvr 3 3 Vaccination Strategies 3.1 Early vaccination, regular booster vaccination and emergency vaccination programs Vaccinating poultry in the early stages of their lives, including egg inoculation, is becoming increasingly common and can provide immunity to poultry either before or immediately after hatching. Compared with vaccinating after hatching, in-egg vaccination involves directly injecting the vaccine into the egg during hatching. This enables the chicks to grow better after hatching, enhances their immunity, and reduces the embryo mortality rate (Chao et al., 2019). This method is particularly useful for the prevention and control of Marek's disease and infectious bursal disease, because early protection is very crucial for the health of poultry flocks (Peebles, 2018). Regular booster vaccinations are crucial for maintaining the long-term immunity of poultry flocks, especially when they are constantly threatened by pathogens. The timing and frequency of booster shots should be determined based on different diseases, vaccine types and breeding models. They need to be carefully arranged so as to achieve the best immune effect and minimize protective loopholes (Soutter et al., 2020). When a disease breaks out, emergency vaccination needs to be initiated to quickly control the spread of the disease. This requires prompt action, usually through large-scale vaccination to protect susceptible poultry (Ravikumar et al., 2022). 3.2 Advantages and disadvantages of injection, drinking water and spray methods Injection vaccination, including intramuscular injection and subcutaneous injection, can ensure the accuracy of vaccine dosage and works well for some vaccines. However, this method is very labor-intensive and may cause stress or harm to poultry (Figure 1) (Peebles, 2018; Saeed et al., 2019). In commercial breeding, intramuscular injection can be operated through automated equipment to evenly vaccinate a large number of eggs before hatching. However, in large-scale flocks of poultry, it is relatively difficult to conduct injection and vaccination after hatching (Peebles, 2018). Figure 1 Application of in ovo technology for various biological supplements in poultry (Adopted from Saeed et al., 2019) Vaccinating poultry through drinking water and spraying is widely used in large-scale farming because it is convenient to operate and can quickly complete the vaccination of a large number of poultry (Renu et al., 2020; Ravikumar et al., 2022). Just like the Salmonella vaccine with chitosan adjuvant (Renu et al., 2020), when fed to poultry through drinking water or mixed feed, it can cause a strong immune response in the mucous membranes and the whole body of the poultry. However, these vaccination methods may result in different doses of vaccine
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