Evaluation of a measles vaccine campaign by oral-fluid surveys in a rural Kenyan district: interpretation of antibody prevalence data using mixture models

Abstract

We evaluated the effectiveness of a measles vaccine campaign in rural Kenya, based on oral-fluid surveys and mixture-modelling analysis. Specimens were collected from 886 children aged 9 months to 14 years pre-campaign and from a comparison sample of 598 children aged 6 months post-campaign. Quantitative measles-specific antibody data were obtained by commercial kit. The estimated proportions of measles-specific antibody negative in children aged 0-4, 5-9 and 10-14 years were 51%, 42% and 27%, respectively, pre- campaign and 18%, 14% and 6%, respectively, post-campaign. We estimate a reduction in the proportion susceptible of 65-78%, with approximately 85% of the population recorded to have received vaccine. The proportion of 'weak' positive individuals rose from 35% pre-campaign to 54% post-campaign. Our results confirm the effectiveness of the campaign in reducing susceptibility to measles and demonstrate the potential of oral-fluid studies to monitor the impact of measles vaccination campaigns.

Fig. 1

Resultant fit of a mixture model applied to measles-specific antibody data from surveys pre- and post-vaccination campaign in Kilifi District, Kenya, 2002. (a–c) Frequency in percentage by antibody reactivity category [with equal-width bands based on log OD T/N values – see main text for explanation)] for pre-campaign raw data (■) and model fit (——) and post-campaign raw data (○) and model fit (- - - -) for age groups (a) 0–4 years, (b) 5–9 years, (c) 10–14 years. (d) The modelled distributions of the positive, weak positive and negative components.

Fig. 2

Comparison of model estimates of (a) measles seronegativity and (b) proportion weakly positive by age group in children pre-campaign () and post-campaign (□) in Kilifi District, Kenya, 2002 (95% confidence intervals are shown).

Fig. 3

Comparison of model estimates of measles seropositivity by age in children aged 1–5 years residing in Kilifi township () and in a rural area, Ngerenya (□), pre-campaign, Kilifi District, Kenya, 2002.