Safe, effective vaccines are available for many serious childhood illnesses. In Australia, vaccines are provided and administered at low or no cost to the recipient and reported immunization rates are high (86–93% in the case of measles). 1–3 Despite the apparent high uptake and efficacy (95% in clinical trials 4,5 ) of measles vaccine, 2373 cases of measles were reported in New South Wales (NSW) in 1993, 1504 were reported in 1994 and 603 cases were reported in 1995. Of these case reports, 11.2% involved children aged 18–60 months (Notification Database, NSW Health Depart- ment, unpubl. data). Among the reasons for the apparent paradox of large numbers of measles cases occurring when reported immuni- zation rates and vaccine efficacy are high are the possibilities that: (i) parental reporting may overestimate immunization rates; 6 (ii) vaccine efficacy may be lower when administered in a clinical practice setting compared with clinical trials (e.g. poor maintenance of ‘cold chain’, suboptimal vaccine administra- tion); and (iii) secondary vaccine failure may be occurring (i.e. immunity waning with time). These factors could also influence the efficacy of the rubella vaccine. The introduction of the Australian Childhood Immunisation Register (ACIR) 7 should lead to improved monitoring of J. Paediatr. Child Health (2000) 36, 418–421 Serological survey of measles and rubella immunity in Sydney preschool children J CAUSER, 1 M MIRA, 1 M KARR, 1 L HUESTON, 3 M BURGESS, 4 G ALPERSTEIN, 2 M FETT 5 and A CUNNINGHAM 3 1 Division of General Practice and 2 Community Health Services, Central Sydney Area Health Service, 3 Virology, Institute of Clinical Pathology and Medical Research, Westmead Hospital, 4 National Centre for Immunisation Research and Surveillance of Vaccine-Preventable Diseases, New Children’s Hospital, Westmead and 5 Public Health Unit for Central and Southern Sydney, Sydney, New South Wales, Australia Objectives: To estimate the prevalence of serological evidence of immunity to measles and rubella in preschool children in central and southern Sydney (NSW, Australia) and the prevalence of immunity in children with either documented or parentally reported immunization. Methods: Geographical cluster random sampling was used to select children aged between 18 and 60 months to partici- pate in the present study. Standardized interviews obtained information on each child’s reported (by parents) immunization status and documentary evidence of immunization was recorded from the Personal Health Record. Venous blood was collected, serum was separated and stored frozen until tested. Measles and rubella antibodies were measured using ELISA, with either immunofluorescence or haemagglutination inhibition being used to clarify equivocal results. The study was conducted from 1992 to 1994 in conjunction with surveys of blood lead concentrations, iron status and micronutrient status. Results: Parents of 726 of 953 children identified between 9 and 60 months of age agreed to participate in the lead, immunization, iron status and micronutrient studies. Sufficient blood for antibody testing was obtained from 580 children, aged 18 to 62 months at the time of collection. Parents reported that 94.7% (95% confidence interval (CI) 92.7–96.5%) of children had received a measles–mumps or measles–mumps–rubella (MMR) immunization. General practitioners adminis- tered 72.8% of these immunizations. The prevalence of serological evidence of immunity to measles and rubella was 88.8% (95% CI 86.2–91.4%) and 91.9% (95% CI 89.6–94.2%), respectively. There was documented evidence of measles and rubella immunization for 88.4% (95% CI 85.7–91.2%) and 86.4% (95% CI 83.4–89.3%) of children, respectively. Of children with documented measles immunization, 91.6% (95% CI 89.2–94.0%) had detectable measles antibody. Of children with documented rubella immunization, 97.2% (95% CI 95.8–98.6%) had detectable rubella antibody. Conclusions: Measles and rubella immunization rates in central and southern Sydney are relatively high and most of these immunizations are provided by the private sector. Immunity to rubella in children with documented rubella immuni- zation is at the level that would be expected from seroconversion studies. Immunity to measles in children with docu- mented measles immunization is slightly lower than expected from seroconversion studies, highlighting the need for the second MMR immunization in preschool children, as well as making near universal immunization imperative if this disease is to be eradicated. Key words: immunization; measles; rubella; seroconversion. Correspondence: Professor M Mira, Division of General Practice, 37 Booth Street, Balmain, NSW 2041, Australia. J Causer, MB BS, MPH, Research Officer. M Mira, BSc(Med), MB BS, PhD, Director, Division of General Practice, Central Sydney Health Service, Clinical Professor, Department General Practice, University of Sydney. M Karr, MPH, MSc(Med), Research Officer. L Hueston, BSc, MSc, Senior Scientist. M Burgess, MB BS, MD, FRACP, FAFPHM, Director and Professor of Paediatrics and Preventive Medicine, University of Sydney. G Alperstein, MB ChB, MPH, FAFPHM, FRACP, Area Paediatrician. MFett, MB BS, BMedSc, MPH, MD, FACOM, FAFPHM, Medical Adviser (Research), Depart- ment of Health and Aged Care, Canberra. A Cunningham, MB BS, MD, FRCPA, FRACP, Director, Westmead Millennium Institute and Research Centre, Westmead Hospital, Professor, Research Medicine, University of Sydney. Accepted for publication 23 March 2000.