"First immunization" refers to the very first dose of a vaccine or medicine given to a person to protect them against a specific disease. It builds the body's protection against the disease, helping to prevent it in the future. Full definition
The colostrum your baby receives in early breastfeeding is important for baby and has been referred to as «baby's first immunization!» (cindyandjana.com)
Colostrum is so full of antibodies, white blood cells, and other immune properties that it's like your child's first immunization. (verywellfamily.com)