Why aftr 2 month injection of bcg vaccine, the baby had blister on the left upper arm?why it took 2 month?

BCG: Bcg is an intradermal injection at the deltoid. If bcg is accidentally given subcutaneously, then a local abscess may form that can ulcerate, and may need therapy with antibiotics. Without treatment it can spread, causing damage to other areas. If pus is aspirated and analysed and treated correctly, the abscess will generally heal on its own in a matter of weeks. It may take time to develop.

Blister after BCG: Thanks for your question. The bcg vaccine contains some preservatives. Sometimes, people can have irritation in their arm from the preservative, and this can be delayed even several months from the vaccine. It's not a dangerous reaction, but i recommend that you take the baby to your primary care physician to have the arm looked at.