Postnatal decline of maternally acquired rubella antibodies.

The postnatal decline of maternally acquired rubella antibody was studied in a large group of infants. A high degree of variability was found in the rate of antibody decline (half-life). Ninety-two babies had rubella antibody half-lives lying between 14 and 70 days and three had values considerably higher. There was no significant difference between the rubella antibody half-lives of the sexes. The antibody titre at birth was weakly correlated with both birth weight and gestational age. There was a highly significant positive correlation between the baby's antibody titre at birth and that of its mother. There was a positive relationship between the half-life and the persistence of rubella antibody. Some babies had no detectable antibody by 2 months whereas others still possessed antibody at 9 months. It was found that the relationship between the half-life and the rubella antibody titre at or near birth could be described by a rectangular hyperbola.