Since the mid-1960s, data for estimating vaccine coverage in England and Wales have been collected from health authorities and published annually by the Department of Health (DH) and the Welsh Office. Estimates initially used the number of live births in each district health authority as the denominator, moving in 1988 to using resident children in the district (Körner returns) (1). These annual data reported vaccinations given up to three years previously and, although they were useful for measuring long-term trends, did not provide information on recent fluctuations in vaccine cover or on the implementation of new vaccines.
In 1984, the World Health Organization set a target of 90% primary vaccination cover for Europe by 1990 in an attempt to eliminate indigenous measles, poliomyelitis, neonatal tetanus, congenital rubella, and diphtheria by 2000 (2). Vaccine coverage for measles and pertussis vaccines in England and Wales at this time fell well short of this target. In the mid-1980s, however, the increasing use of computerised child registers that hold vaccination details for all children meant that vaccination coverage data for the entire population could now be calculated using resident children as the denominator. In addition, designated co-ordinators of local vaccination programmes were appointed. These two developments created the opportunity, through the development of the COVER (Cover of Vaccination Evaluated Rapidly) programme, to improve coverage by providing rapid feedback of clear, accurate and relevant information and to enable changes in vaccine coverage to be detected quickly (2).
COVER was first piloted in England and Wales in January 1987, with 14 districts contributing data. By May 1989, 175 of the 200 districts in England and Wales were participating. Vaccination data were requested from each district/health board in England, Wales and Northern Ireland for quarterly cohorts of resident children who had recently reached target age for completing vaccination with sentinel antigens. District data was aggregated at the Communicable Disease Surveillance Centre (CDSC), and regional and national statistics fed promptly back to district immunisation coordinators and regional medical officers. Summary data were also included in the Communicable Disease Report (CDR).
Concurrent to COVER data being collected by CDSC from districts in England, Wales and Northern Ireland each quarter, Körner data were compiled annually for England by DH, for Wales by the Welsh Office, and for Scotland by the Scottish Information and Statistics Division for the Scottish Office. Although there were great similarities in these datasets they were not directly comparable, resulting in a duplication of effort, and the publication of slightly conflicting data. In 1995, the Körner and COVER systems were merged to create an integrated and more streamlined system for the collection of immunisation coverage statistics (3). At the same time compatible data from Scotland became available allowing the production of UK coverage statistics.
With the reorganisation of the NHS in April 2002, COVER data for England is now collected from primary care trusts (PCTs) rather than health authorities. The denominator is defined as the PCT relevant population and includes all children registered with a GP whose practice forms part of the PCT, regardless of where the child is resident. It also includes any children not registered with a GP, who are resident within the PCT's statutory geographical boundary. Children resident within the PCT geographical area, but registered with a GP belonging to another PCT, are the responsibility of that other PCT.