Measles immunity and immunisation status in under-5-year-old children in New South Wales: A population-based study

Objective: To estimate the proportion of 1–4‐year‐old New South Wales children immune to measles and compare the documented immunisation history with serologically defined immune status. Design: Population based seroprevalence survey piggybacked onto the National Survey of Lead in Children. Immune status was determined by two different enzyme immunoassays on plasma samples from subjects. Setting: New South Wales, February‐March 1995. Outcome measures: Documented measles immunisation collected by interview survey and serologically defined immunity. Results: Of 689 survey subjects, 430 (62.4%) provided a blood sample. Adequate plasma remained for both assays for 347 children, of whom 279 (80.4%) were immune by both assays. Parents of 330 stated that their children were immunised, of whom 211 (63.9%) were able to produce corroborating records. Of these 211 subjects, 178 (84.4%) were immune compared to 87 (76.3%) of 114 without records (P=0.07). Conclusions: We estimate the prevalence of true measles immunity in 1–4‐year‐old NSW children to be only 80%, a level inadequate to prevent outbreaks of measles in urban populations. Both long term and immediate strategies are required to increase the prevalence of immunity among NSW children; these may include lowering the age of the routine second measles dose and mounting a mass measles immunisation campaign to include preschool aged children.