FIG. 4.

The ability of vaccinia virus to rapidly cross the placenta is dependent upon the initial dose of infection. Pregnant female mice were infected intraperitoneally 14 days postconception with 6 × 10⁶ PFU of VVWR. Viral titers in the uterus, placenta, and fetuses were determined 4 days later by plaque titration as described in Materials and Methods. The dotted horizontal line indicates the limit of detectability of the plaque assay (120 PFU/g of tissue). The maternal virus titer in the uterus is shown as a solid horizontal line.