An effectively attenuated vaccine still infects cells in the host but is unable to cause severe illness due to a reduced ability to invade cells or evade the immune system. The vaccine strain can replicate to some degree, expressing enough of its antigenic proteins to trigger a strong immune response. https://hallbook.com.br/blogs/491005/Life-Attenuated-Vaccines-An-Effective-Way-to-Build-Immunity-Against