Background: We examined risk factors for measles infection before measles vaccination at 9 months of age in Guinea-Bissau.
Methods: Among 1524 children enrolled in a trial of early measles vaccination at 4.5 months of age, we assessed the relative risk (RR) of measles before enrollment and the incidence rate ratio between 4.5 and 9 months of age for different groups.
Results: The incidence was high, with 4% having measles before 4.5 months and 10% having measles between 4.5 and 9 months of age. The main risk factor was the age of the mother; children of young mothers (age, 15-24 years) had lower antibody titers and higher risk of measles than children of older mothers both before 4.5 months (RR = 1.74 [1.02-2.96]) and between 4.5 and 9 months of age (incidence rate ratio = 1.59 [1.05-2.41]). Having no Bacillus Calmette-Guérin scar was associated with a higher risk of measles before 4.5 months of age (RR = 2.61 [1.54-4.45]). Children who were not breast-fed and had fever or respiratory infection at enrollment had a 2- to 4-fold higher risk of measles between 4.5 and 9 months of age.
Interpretation: Young mothers transmit lower titers of antibodies to their children and an increasing proportion of infants become susceptible to measles before the age of measles vaccination.