MMR uptake data are unlikely to be subject to manipulation

Editor—Scanlon suggested that uptake of measles, mumps, and rubella (MMR) vaccine is lower than reported because of manipulation of target groups by general practitioners.¹ The figure of 84% quoted, however, comes from the national “coverage of vaccination evaluated rapidly” scheme.² These data derive from computerised child health registers in each health authority and not from target payments.³

The denominator includes all children who are resident on the last day of each quarter, regardless of whether they are registered with a general practice. Children are entered on to the system at birth (from the statutory birth notification) and on movement into the area, usually by the health visitor. The numerator is the number of those children who had received the vaccine by their second birthday.

Vaccination coverage according to this scheme is lower than data derived from target payments because of the inclusion of unregistered children; because of the failure to remove children who move out of the area; and because data on vaccines given are sometimes not returned. (This may happen because the vaccination has been given at a time other than the scheduled appointment, at another clinic, or in private practice or because the practice has refused to return information.)

In 1997 we reviewed eight unpublished audits of data held on the child health systems. The audits suggested that our data underestimate true uptake by between 1% and 9% in children assessed before their third birthdays. The greatest underestimates were in districts with low coverage and high population mobility.

Although the child health systems tend to underestimate vaccine coverage, the scheme has advantages over target payment data. Quarterly evaluation of uptake for each antigen at exactly 1, 2, and 5 years reflects coverage according to the recommended schedule rather than the less timely definitions in the target payment system. The data are therefore available for monitoring trends in coverage with time and comparing the differences between antigens.⁴ The latest data show that uptake of measles, mumps, and rubella vaccine at age 2 has declined by around 8% since 1995 and is about 10% lower than uptake of other primary vaccinations.²

Ongoing changes in primary care are likely to weaken the role of target payments as performance indicators. Coverage data will be available from the child health systems and should be used for performance management and for public health. Primary care teams need to work with immunisation coordinators and community paediatricians to ensure that during the changes this important source of information is not lost.