Early Waning of Maternal Measles Antibody in Infants

Abstract

Source: Leuridan E, Hens N, Hutse V, et al. Early waning of maternal measles antibodies in era of measles elimination; longitudinal study. BMJ. 2010; 340: C1626; doi: 10.1136/bmj.c1626Investigators from Belgium recruited healthy mother-infant pairs to examine the presence of measles IgG antibody, from week 36 of pregnancy until 12 months of age. Antibody levels from infants and their mothers who were immunized for measles but had no history of measles disease were compared with levels in infants and their mothers who had a past history of measles infection but no measles vaccination. Decay of maternal antibody in the infants over time was followed and modeled. A “threshold” level of antibody (≥300mIU/ml) was considered “protective.”Data from 207 women (ages 18–40 years) and their 210 infants were analyzed. Of these women, 120 were disease-immune and 87 had been vaccinated. At 36 weeks gestation, 8% of disease-immune women versus 26% of vaccinated women had antibody levels that did not reach the “protective” threshold level. The geometric mean titers (GMT) in disease-immune and vaccinated women differed significantly, with mean values of 2,687 mIU/ml and 779 mIU/ml, respectively. At 3 months of age, only 29% (21/73) of infants of vaccinated mothers had a “protective” level for measles antibody versus 60% (51/850) of infants from disease-immune women. By age 6 months, only 1 of 31 infants from vaccinated mothers had a “protective” level of measles antibody compared with 10 of 41 infants of disease-immune mothers. By 9 months of age, no infant in either group had a measles antibody titer ≥300mIU/ml.With the use of a linear mixed model, and accounting for a variety of interactions, time, and antibody concentrations in the mother, the median time to loss of protective levels of passive measles antibody was calculated to be 3.78 months in infants of disease-immune mothers versus 0.97 months for infants of vaccinated mothers.Because routine immunization of children against measles takes place on or after 12 months of age, the results of this study demonstrate an increasing gap of susceptibility between loss of maternal antibodies and timing of routine infant vaccination. The authors conclude that their findings reinforce the need for timely measles immunization.This study is consistent with many others1–5 that demonstrated that measles antibody passively transferred to infants from mothers immunized against measles is lower in titer and doesn’t persist as long as antibody from women who had measles disease. While this study showed the median time to loss of measles immunity was 0.97 months for infants of vaccinated women and 3.78 months for infants of mothers who were disease-immune, previous studies using plaque neutralization titers (PNT) have shown immunity to measles (PNT ≥1:8) may last seven to nine months, and rarely as long as 12 months in those infants born to disease-immune mothers. While immunity from measles disease and, probably, vaccine immunity are presumed to be life-long, passive immunity in an infant is dependent upon amount of antibody transferred and the decay of that antibody over time. In a population that has no ongoing exposure to measles disease, levels of detectable measles antibody may decline over time in both disease-immune and immunized individuals. As measles elimination progresses worldwide, mothers may transmit to their infants antibody protection that lasts only a few months.The current study reminds us that as more and more mothers become measles-immune through immunization and not through measles disease, their infants are likely to be susceptible to measles at an earlier age. Current immunization schedules, which recommend routinely immunizing children ≥12 months of age, are based on data from infants of disease-immune mothers and may need to be re-examined. Avoiding any delay in measles immunization is important to prevent resurgence of measles disease, especially among infants.Although at some point in the future the recommended age for the first dose of measles vaccine may need to be reconsidered, today the majority of recent US measles cases occur in non-vaccinated, vaccine-eligible individuals. In 2008, of the 127 reported cases, 67% were among those exempted from vaccination due to their personal beliefs. Only 17 cases occurred in children too young to be routinely vaccinated.6