The use of COSAS in the analysis of vaccination coverage in urban, peri-urban and rural populations in the Edendale/Vulindlela district of KwaZulu. 
A repeat vaccination coverage survey has been conducted in the Edendale/Vulindlela district of KwaZulu.  The survey data were processed using the Coverage Survey Analysis System (COSAS) developed by the World Health Organisation (WHO) through its Expanded Programme on Immunisation (EPI).  A modified random cluster sampling method was used to select 281 children between the ages of 12 and 23 months.  Of the children surveyed, 83% were in possession of Road-to-Health cards (RTHCs).  The best estimate of overall coverage for doses up to and including the second doses of polio and diphtheria, pertussis and tetanus (DPT) was 85% or higher, but estimates for polio 3 and measles, at 72% and 67% respectively, remain suboptimal.  Stratification of coverage into urban, peri-urban and rural categories revealed that the major contribution to the fall-off in coverage, after the second dose of polio and DPT, came from children in the peri-urban category with estimates of 52% for polio 3 and 38% for measles.  The fact that coverage in the peri-urban population for doses up to and including polio 2 was 78% or higher indicated that the peri-urban influence responsible for this drop-out effect occurred between the approximate ages of 5 and 8 months.  This identified populations in informal peri-urban settlements as a priority group for urgent intervention and further study.  The estimation of missed opportunities at visits when vaccinations are normally given, found in this survey to occur in 17% of children, was a useful feature of COSAS and provided a basis for a specific intervention.