According to the world literature, the frequency of complications of BCG vaccination varies in different regions from 0.004% to 2.5% of cases. The increase in the frequency of complications of BCG-etiology is associated with the biological properties of the BCG strain, the violation of the technique of administration of the vaccine, the change in the reactivity of the immune system of the vaccine. Newborns with primary immunodeficiency states that have significant defects in cellular immunity in the absence of clinical signs of the disease, from birth or from an early age are susceptible to infections and accordingly have a higher risk of developing complications of vaccination. According to literary sources, the introduction of the BCG strain itself may be affected by the change in immunological reactivity, resulting in the development of the postvaccine secondary transient T-cell immunodeficiency, which proceeds heavier than the more immunocompromised individuals it develops. Therefore, careful analysis of the child’s health status is required both before BCG vaccination and after immunization, given that the majority of the vaccinees have a marked secondary immunodeficiency.