Serological response to early measles vaccination.

This study compares the persistence of measles IgG antibody in 239 children vaccinated at 6-8 months of age with 76 children vaccinated after 8 months of age. Among the children vaccinated prior to 9 months, 49 per cent of the children between 16 and 44 months and 33 per cent of children over 54 months had levels of measles IgG antibody conventionally considered protective. Among the children older than 48 months, 67 per cent of children vaccinated before 9 months and 13 per cent of children vaccinated after 8 months had antibody levels below the conventionally accepted protective levels of 0.2 IU/ml. Older children had lower antibody levels than younger children. Measles immunization before 9 months with the standard titer Edmonston-Zagreb vaccine has not provided a large proportion of under-five children with protective levels of measles IgG antibody. A significant proportion of children vaccinated at the currently recommended age also had suboptimal levels. It is difficult to protect the majority of the measles-susceptible population with a single dose regardless of the immunization schedule used. A second dose of measles vaccine may be necessary to increase the herd immunity.