The consequences of Calmette-Guérin bacillus (BCG) vaccination were followed in newborn and adult mice.

BCG failed to multiply in the organs of adult mice when administered peritoneally. In contrast, extensive multiplication of the vaccine occurred in both splenic and pulmonary tissue after its peritoneal administration to newborn mice. This absence of tuberculostasis occurred during the period when the animal's spleen, lung, and thymus were rapidly growing.

Animals achieved similar levels of resistance to virulent respiratory challenge 10 wk after vaccination irrespective of whether the vaccine had been administered when the mice were newly born and BCG had multiplied in vivo or administered when the animals were fully grown and the BCG had remained in the dormant state.

Although neonatal infection with BCG was severe, as shown by the large numbers of organisms recovered from the animals' tissues, the animals suffered no mortality or overt signs of disease. Neonatal vaccination did not significantly affect either the animal's growth rate or the gross development of its organs.

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