Girls may have lower levels of maternal measles antibodies and higher risk of subclinical measles infection before the age of measles vaccination

Abstract

Background

Previous studies have suggested that girls may have lower maternal measles antibody levels than boys. Girls might therefore be more likely to contract measles infection before the normal age of measles vaccination at 9 months of age.

Methods

In connection with a clinical trial of different measles vaccination strategies, we collected pre-measles vaccination blood samples at 4.5 months of age from two subgroups of children. Samples from these children were used to assess possible differences in maternal antibody levels for boys and girls. At 9 months of age another subgroup of children was sampled before the normal measles vaccination; these samples were used to assess the frequency of subclinical measles infection among boys and girls.

Results

We determined measles-specific antibody levels for 812 children at 4.5 months of age and for 896 children at 9 months of age. At 4.5 months of age girls were less likely to have protective maternal antibody levels, the male–female ratio for protective antibody level being 1.23 (1.00–1.51). Among children sampled at 9 months of age, girls were more likely to have protective levels, the female–male ratio for having protective antibody levels being 1.65 (0.98–2.78) (p = 0.054) and the geometric mean titre was significantly higher for girls (p = 0.007). Children who lived in houses with known measles cases were more likely to have protective levels at 9 months of age even though they had not reported measles infection. Since we had excluded children with known measles infection, girls may have been more likely to have had subclinical measles infection. Combining clinical and possible subclinical measles infection, girls tended to be more likely than boys to contract measles infection before 9 months of age, the RR being 1.36 (0.97–1.90).

Conclusions

Girls lost maternal measles antibodies more rapidly than boys and well before 9 months of age. They may be more likely to contract subclinical measles infection before the current age of measles vaccination.

Introduction

The measles-specific maternal antibodies that children receive at birth protect them from the disease in the first month of life. However, this protection depends on factors that affect the concentration of antibodies in the mother, and the foetal transfer. Factors such as high parity, maternal age, prematurity, maternal HIV infection, syphilis and placental malaria infection, influence negatively the transfer of measles antibody to the child [1], [2], [3]. As a consequence, some children have a shorter period of protection during the early months of life [4], [5]. Studies from West Africa have suggested that girls may have lower measles-specific maternal antibody levels [6], [7]. This could partly explain why girls are often found to have higher measles-specific antibody levels after immunisation [8]. There are some indications that this may also have been important in relation to the reduced survival after the high-titre Edmonton-Zagreb (EZ) vaccination since mortality was apparently higher among children with low antibody levels at the time of vaccination [9].

If girls have lower maternal antibody levels they might be at greater risk of early measles infection. In connection with a randomised trial of early measles vaccination at 4.5 months of age followed by a normal measles vaccination at 9 months of age we collected pre-vaccination blood samples at 4.5 and 9 months of age [7]. Since a measles epidemic occurred during the conduct of the trial, we had the possibility of examining whether the risk of clinical and subclinical measles infection differed among the children who had provided a sample for antibody analysis.