Determining when to vaccinate and which vaccines to use is a major concern for poultry farmers, especially large-scale operations. It is important to recognize that no universal immunization program exists. Blindly adopting another farm's program may not achieve optimal results in your own flock. The only effective approach is to develop a farm-specific program based on fundamental immunological principles, tailored to local conditions and informed by others' successful experiences. When formulating an immunization program, the following key factors should be prioritized:

1. Local disease prevalence  

2. Farm disease history and current major infectious disease threats  

   - Vaccination against diseases not yet diagnosed on the farm should only be considered when there is a confirmed serious threat.  

   - Use of virulent vaccines (e.g., virulent infectious laryngotracheitis vaccines) requires extreme caution and should be avoided unless absolutely necessary.

3. Poultry purpose and rearing period  

   - Example: Breeding chickens require inactivated infectious bursal disease oil-emulsion vaccines before laying, whereas commercial layers do not.

4. Disease status in the region of origin of the chicks  

5. Maternal antibody interference  

   - Critically affects vaccine selection and timing of initial vaccination for diseases such as Marek's disease, Newcastle disease, and infectious bursal disease.

6. Disease resistance variations among different species and breeds of poultry  

7. Interference between different vaccines  

8. Selection of vaccine strain serotype, subtype, or specific isolate  

9. Choice of vaccine formulation  

   - Decisions between live or inactivated vaccines, wet or freeze-dried vaccines, cell-associated versus non-cell-associated vaccines, etc.

10. Selection of vaccine country of origin and manufacturer  

11. Determination of vaccine dosage and dilution volume  

12. Choice of vaccine administration route  

13. Combined use of certain vaccines  

14. Sequential use of vaccines from the same type with increasing virulence  

    - Example: Use of IB vaccine H120 before H52.

15. Sequential use of live vaccines followed by inactivated oil-emulsion vaccines for the same disease  

16. Necessary adjustments based on immunization monitoring results or disease outbreaks