How Important is the Second Measles Immunization?

Abstract & Commentary

Synopsis: Of 130 children who were seronegative after one dose of measles vaccine, 106 children became seropositive after a second dose 4-11 years later, meaning that more than 98% of all children had antibodies after two immunizations.

Source: Poland GA, et al. Measles reimmunization in children seronegative after initial immunization. JAMA 1997; 277:1156-1158.

The immune response to the two-dose measles immunization strategy was studied prospectively in 1490 children. Of 130 children who were seronegative after one dose of measles vaccine, 106 (81.5%) became seropositive after the second dose given 4-11 years later. Lack of response was associated with age of initial measles vaccination less than 13 months and with the increasing interval between the two doses. The earlier timing of measles reimmunization at 4-6 years may be preferable to reimmunization at 11-12 years.

COMMENT BY HAL JENSON, MD, FAAP

Following a nadir in measles cases in the United States in 1983, numerous outbreaks in the late 1980s, with 20%-40% of cases occurring in persons who had been age-appropriately immunized, led the Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics to recommend a second dose of measles vaccine (as MMR) for all children. This study is one of the largest reimmunization studies using the post-1980 Moraten vaccine, the measles vaccine currently used in both the united States and Canada. Although the seroconversion rate of 81.5% to the second dose of measles vaccine among nonresponsers appears to be only moderately successful, the overall response rate for the entire cohort to the two-dose regimen was 98.2%. This is sufficient for herd-immunity to measles to occur, and it supports the current recommendations for the two-dose measles immunization schedule.

The second measles dose is not a booster dose, but it is intended to overcome vaccine failure and to ensure that all children receive at least one dose of vaccine. In this study, seroconversion was equivalent for children with negative (89 of 106; 84%) or equivocal (17 of 24; 71%) baseline serology. Lack of seroconversion after reimmunization was associated with younger age and with increasing time since initial measles immunization. Passively transferred maternal antibodies are protective against measles, but they also diminish the efficacy of measles immunization during the first year of life. Now that the cohort of women of child-bearing age includes mostly women with vaccine-induced measles immunity instead of immunity following natural infection, most offspring receive relatively lower levels of measles antibody than in the pre-immunization era. This is the basis for the recent reduction from 15 months to 12 months for measles vaccination of infants. This study demonstrates that initial measles immunization at less than 1 year also has a negative effect on antibody response to the second vaccination given years later. The immunologic phenomena that account for this are unclear.

Another interesting finding of this study is the lower liklihood of seroconversion with increasing time since the initial measles vaccination (P = 0.001). This suggests that the time of the second measles vaccination should be at 4-6 years rather than at 11-12 years. (Dr. Jenson is Chief of Pediatriac Infectious Diseases, University of Texas Health Science Center, San Antonio, TX.)