Waning of Maternal Antibodies against Measles Suggests a Large Window of Susceptibility in Infants in Lao People's Democratic Republic

Abstract

Introduction: Measles is an endemic but largely neglected disease in Lao People's Democratic Republic. New-borns are protected by maternal antibodies, but antibody waning before measles vaccination at 9 months of age leaves infants susceptible to infection. In this study, the susceptibility window of infants was determined to generate scientific evidence to assess the national measles immunization strategy.

Methods: Between 2015 and 2016, demographic data, medical history, and blood samples were collected from 508 mother-child pairs at the provincial hospital in Vientiane. The samples were screened with a commercial kit detecting anti-measles IgG antibodies.

Results: The large majority (95.7%) of the mothers were seropositive for anti-measles IgG and antibody titers of the mothers and infants were highly correlated (p < 0.01). While at birth 97.7% of the infants were seropositive, seropositivity rates decreased to 74.2% two months later to reach only 28.2% four months after birth (p < 0.01). Just before the first dose of the measles-rubella vaccine, scheduled at 9 months of age, was actually given, less than 14% of the infants were seropositive.

Conclusion: This alarmingly wide susceptibility gap due to rapid maternal antibody decay leaves infants at risk of measles infection and serious disease complications. A high herd immunity is crucial to protect young infants and can be achieved through improved routine vaccination coverage and (expanded age group) supplementary immunization activities.

Keywords: Lao PDR; maternal antibodies; measles; seropositivity; waning.

Figure 1

Correlation between log-transformed antibody titers of mothers and infants at birth and the effect of age of the mother (years). The blue line is the linear regression curve and the confidence interval is shaded (Adj R2 = 0.88, Intercept = 0.15, Slope = 0.96, p < 0.001).

Figure 2

Waning of maternal antibodies after birth. Scatterplot of the corrected delta optical density (c∆OD) and the age (months) of the child. The different symbols represent the different sampling time-points. The colored large dots show the decreasing seropositivity rates at various time points: the green dot being the seropositivity rate at birth (in cord blood samples), the red dots being the seropositivity rates at months 2, 3, 4, and 10 (in dried blood spot samples-DBS), and the full and empty blue dots being the seropositivity rates at months 2, 3, 4, and 10 (in DBS) when applying an adjustment factor of 1.94 and 2.13, respectively. The blue line is the LOESS curve. Shaded are the confidence intervals. The horizontal black line is the cut-off value for c∆OD used in the ELISA to differentiate seropositives/equivocals from seronegatives for anti-measles antibodies (0.1).