SUMMARYWe conducted a community-based cluster sample survey of rubella sero-epidemiology in Addis Ababa, Ethiopia in 1994. Among 4666 individuals for whom complete data were available, rubella antibody prevalence was 91 % (95 % confidence interval : 90, 92). On multivariable analysis, seroprevalence was lower among individuals who were resident in Addis Ababa for 1 year or less. Approx. 50 % seroprevalence was attained by age 4 years, and the estimated average age at infection was 5n2 years. The highest age-specific force of infection was estimated to occur in 5-to 9-year-olds. The early age at infection corresponded with a low estimated incidence of congenital rubella syndrome (CRS) of 0n3 per 1000 live births, equivalent to nine cases of CRS in 1994. The predicted critical level of immunity for elimination of rubella via vaccination was 85-91 %, requiring 89-96 % coverage with a vaccine of 95 % effectiveness. Unless very high coverage of rubella vaccine could be guaranteed, the introduction of childhood vaccination could increase the incidence of CRS in Addis Ababa.

A measles outbreak in December 1998 in Bedelle (vaccine coverage <40%) and two sporadic cases in Addis Ababa, Ethiopia, were investigated. Paired serum and oral fluid samples were collected 2-8 days after the onset of symptoms. A total of 53 of 55 outbreak cases and both sporadic cases were positive for serum measles virus-specific IgM. Oral fluid measles-specific IgM was positive in 71% of cases collected up to 5 days after onset and in 90% collected at 6-8 days. By contrast, 100% of oral fluid samples were positive for measles virus RNA by RT-PCR, suggesting that early collection of samples favoured the detection of measles virus RNA by RT-PCR. The measles virus strain in the outbreak was identified as genotype D4. One strain from a sporadic case was also genotype D4; the strain from the other sporadic case was assigned to clade D but was distinct. The degree of divergence from recognised clade D strains suggested that, together with three strains from the United Kingdom, it represents an additional genotype of clade D (GenBank accession numbers AF280800-280807).

SUMMARYA method for the analysis of age-stratified antibody prevalence surveys is applied to a previously reported survey of antibody to rubella virus using oral fluid samples in which the sensitivity of the assay used was shown to be compromised. The age-specific distribution of the quantitative results of antibody tests using oral fluids is modelled as a mixture of strong positive, weak positive and negative components. This yields maximum likelihood estimates of the prevalence at each age and demonstrates that, when used in conjunction with mixture modelling techniques, the results of antibody prevalence studies using oral fluids accurately reflect those obtained using sera.