Ulceration is the most common side effect post BCG vaccination. Following vaccination, the local tissue response begins in 2-3 weeks, with scar formation and healing within 3 months. Side effects most commonly are ulceration at the vaccination site and regional lymphadenitis. Some vaccine strains also rarely cause osteomyelitis. In immunocompromised states, disseminated BCG infection can occur. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition