Assay of neutralizing antibody against variola virus by the degree of focus reduction on HeLa cell cultures and its application to revaccination with smallpox vaccines of various potencies.

A method for assaying neutralizing antibody against variola virus was established by focus counting on HeLa cell cultures. The ND(50) titre, i.e., the serum dilution endpoint to give a 50% reduction in the number of foci, was determined with excellent reproducibility.Groups of students 19-20 years of age were revaccinated by the multiple pressure method with serial 10-fold dilutions of a smallpox vaccine and their neutralizing antibody response was assayed by the focus counting assay system and was related to the local skin reactions on the seventh day after inoculation and to the potency of the vaccine administered. There was a significant rise in the antibody level even after inoculation with a vaccine whose potency was as low as 1.3 x 10(5) pock-forming units/ml. In general, the rise in the log antibody level was proportional to the diameter of the reddening, but a significant rise was found among individuals who had no detectable skin reaction. The skin reaction was greater among individuals with a lower initial antibody level when the vaccine administered had a potency lower than 1.3 x 10(6) pock-forming units/ml.