TRANSACTIONSOF THE ROYAL SKIETYOFTROPICAL MEDICINEAND HYGIENE (1995) 89,119-122 119 Prevalence of measles antibody among children under 15 years of age in Santa Cruz, Bolivia: implications for vaccination strategies F. T. Cutts’, A. Bartoloni’, P. Gugliehnetti3, F. Gi14, D. Browns, M. L. Bianchi Bandiiell6 and M. RoseUi2 ‘London School of Hygiene and Tropical Medicine, London, UK; 21nstitute of Infectious Diseases, University of Florence, Florence, Italy; 31n.stitute of Infectious Diseases, University of Siena, Siena, Italy; 4Department of Health, Santa Cruz, Bolivia; %-us Refmxce Division, Central Public Health Laboratory, London, UK; 6Department of Molecular Biology, Division of Microbiology, University of Siena, Siemz, Italy Abstract We conducted a community-based survey in Santa Cruz city, Bolivia, to determine the age-specific pre- valence of measlesantibodies, determine factors associatedwith absenceof detectable measlesantibodies, and to compare results of salivary and serum measles immunoglobulin G (IgG) antibody assays. Serum samples from 1654 children were assayed for measles IgG antibody using the haemagglutination inhibition (HI) assay, and salivary samples were also obtained from 187 children and tested for measlesIgG antibody using an antibody capture radioimmunoassay. Reported measlesvaccine coverage in children aged 12-35 months was 77% (95% confidence interval [CI], 72-81%). Eighty-seven percent (95% CI SS-89%) had de- tectable HI antibody, but a high proportion had antibody levels below 200 miu’(30-40% of 2-14 years old children). Measles seronegativity was associatedwith not being vaccinated against measles, a negative his- tory of measlesdisease, living in the inner city, being a lifetime resident of Santa Cruz, and young age. Of 212 children without detectable measles antibody, 58% had a positive history of vaccination or measles dis- ease,so that historical information was not sufficiently reliable to identify susceptibles. The salivary measles antibody assaywas not sufficiently sensitive to be used for population screening; only 54% of 171 salivary samples from children who had detectable serum HI antibody were positive. A mass measlesvaccination campaign of all children under 15 years of ageis planned in Bolivia in 1994. Although only 7% of school-age children in Santa Cruz were seronegative, the effectiveness of a masscampaign in this agegroup depends in part on the responseto revaccination of children with low, but detectable, antibody levels. Keywords: measles, seroprevalence, vaccination, Bolivia Introduction Measles vaccination is one of the most cost-effective health interventions available, and measles has been identified as a ootential candidate for eradication (HOP- KINS et al., 1982). An increasing number of countries have established targets for the elimination of measles transmission nationally or regionally, and the design and evaluation of optimal- strateiies fdr- using measles vac- cines is a nrioritv research area (NOKES & CUTTS. 1993). Several-Latin American countries have recently con- ducted a mass vaccination campaign of all children aged 9 months to 14 years, and the impact on measles in- cidence has been dramatic. Although many older children are already immune, massrevaccination is more effective than selective vaccination of children classified as susceDtible through a negative bistorv of disease or vaccinadon, becausethese h&tories are ndt reliable (PRE- BLUD et al.. 1982: SCOTT et al.. 1984). The develooment of improved rapid field techri;ques io measure ieasles antibody, perhaps using non-invasive techniques such as salivary assays (PERRY et al., 1993), would enable develo- ping countries to monitor vaccination programmes more precisely and to predict the need for supplementary strategies such as masscampaigns. We conducted a serosurvey in SantaCruz city, Bolivia, to determine the age-specific prevalence of measlesanti- body and compare results of salivary and serum measles immunoglobulin G (IgG) antibody assays, to compare vaccine coverage and seropositivity rates in the inner and the outer city, and to determine factors associatedwith the absence of detectable measles antibodies. Materials and Methods The setting Bolivia routinely administers measlesvaccine at a rec- ommended age of 9 months at health centres, and sup- plements this by annual campaigns involving selective vaccination of children 9-59 months of agewho have not previously received measlesvaccine. In Santa Cruz city, according to routine vaccination reports, it was estimated Address for correspondence: Dr Felicity Cutts, Communicable Disease Epidemiology Unit, London Schoolof Hygiene and Tropical Medicine, Keppel Street, London, WClE 7HT, UK. that 73% of children had received measles vaccine by their first birthday in 1992, and 84% in 1993 (Santa Cruz regional health authority, unpublished data). Of 1314 measlescasesreported by city health facilities in 1992- 1993,49% were in children over 5 years of age. Suruey design We conducted a community survey in Santa Cruz, stratified by inner city (estimated 1992 population 372 276) and outer suburbs (population 322 340). In each stratum, a cluster sample of 20 administrative units (Unidades Vecinales, UV) was selected by sampling with probability proportional to estimated size. We used the detailed maps which are available of the inner city to se- lect randomlv 4 blocks from each UV in the samde. For the outer city, we obtained a list of blocks from’ the ad- ministrative authorities in each UV and selected a ran- dom sample of 4 blocks. In each block, from a random starting point, interviewers proceeded in a clockwise di- rection, visiting every household, and registering every child under age 15 in the household until 10 children had been registereh. Up to 2 revisits were made if a child was not at home. At each household with a child aged O-14 years, we ad- ministered a short questionnaire to obtain information on vaccination status and measles disease. With parental consent, we collected a finger-prick blood sample on ab- sorbent paper (Ml4 0.05 mL microdiluter delivery tes- ter). We collected saliva samplesfrom a subsample of 192 children (all children registered on the last 2 d of the sur- vey), using OraSureTM saliva collection devices (Epitope Inc, Beaverton, Oregon 97005, USA) according to the manufacturer’s instructions. The estimated sample size was 1600 children: 800 in each stratum. With an expected overall measlesseroposi- tivity rate of around 85%, this sample size was adequate to estimate the true rate for Santa Cruz with a 95% con- fidence interval of f4%, assuming a design effect of 4. The sample size had 80% power to detect a 10% dif- ference in seropositivity rates between the inner and outer city areas. Serology Blood samples were left to dry at room temperature,