HealthDay News — In an elimination setting, most infants are susceptible to measles by age 3 months, according to a study published online Nov. 21 in Pediatrics.

Michelle Science, MD, from The Hospital for Sick Children in Toronto, and colleagues selected sera obtained at a tertiary pediatric hospital from infants <12 months born at ≥37 weeks of gestation. Infants with conditions that affect antibody levels were excluded. Sera from 8 predetermined age bands were selected and tested for measles-neutralizing antibody using the plaque-reduction neutralization test. At each age band, the proportion immune was calculated and predictors of infant susceptibility were assessed.

Overall, 35% of the 196 infant sera were from infants with underlying medical conditions. The researchers found that 20% of infants (5 of 25) had antibodies below the protective threshold in the first month, and this number increased to 92% (22 of 24) by 3 months. All infants had titers below the protective threshold by 6 months. Infant age was the strongest predictor of susceptibility in a multivariable analysis (odds ratio, 2.13 for each additional month increase).

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“These findings have important implications for current infant postexposure prophylaxis recommendations because infants <6 months of age are often considered routinely immune to measles,” the authors write. “Further research is needed prospectively to validate these findings and explore the impact of maternal age and breastfeeding on infant immunity in elimination settings.”

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