The protective substances produced by vaccinia virus in swine are transmitted repeatedly to the young by way of the colostrum of the sow. In 2 instances suckling immunity was demonstrable in the young of 6 successive farrowings which numbered 37 and 57 individuals, respectively.

The immunity acquired by suckling began to decline during the 2nd month and was practically negligible by the end of the 3rd month. Vaccinia virus introduced cutaneously during the first few days of life in the passively protected pigs exerted little or no immunizing effect.

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