In vivo vaccination involves injecting the vaccine into which site?

• A. Pullet
• B. Embryo in the egg
• C. Hen's uterus
• D. Newborn chick

Answer: (B)

In poultry vaccination, delivering the vaccine into the living embryo inside the egg is a common method used to immunize chicks before they hatch. This approach, often called in ovo vaccination, lets hatcheries vaccinate large numbers rapidly and primes the chick’s immune system early.

The embryo itself inside the egg is the site of injection, which is why embryos in the egg are the correct choice. Vaccinating a pullet or newborn chick occurs after hatch through other routes, and the uterus isn’t a routine vaccination site in this context. So the embryo in the egg best represents in vivo (in-egg) vaccination.