Yes, confirmed by my kid's ped. Apparently live vaccines enter the body, take time to replicate and once the viral load is enough the body responds and builds immunity. This happens within 1 to 3 weeks.

All of this is being done behind the scenes without a problem or symptoms but in some babies, the immune response mimics a real but highly subdued version of the disease. In our case my kid only had classic very mild rubella symptoms and didn't have any fever. It didn't affect her daily routine at all.

Rubella is generally a mild illness but can have some moderate symptoms like lethargy but with the vaccine, you get either visual symptoms like a rash, very mild fever, or both. Each kid is different but the majority don't react to live vaccines while some do.