The incidence of chickenpox in the community. Lessons for disease surveillance in sentinel practice networks.

Sentinel practice networks have been established in many European countries to monitor disease incidence in the community. To demonstrate the value of sentinel networks an international study on the incidence of chicken pox has been undertaken. Chickenpox was chosen as an acute condition for which incidence data are important to the determination of health policy on vaccine use. The project examined the incidence of chickenpox reported in sentinel networks in England and Wales, The Netherlands, Portugal and Spain (two regional networks) in January-June 2000 and the potential underestimate from patients who did not consult. An investigation of secondary household contact cases was undertaken. Reported incidence of chickenpox (all ages) in England and Wales was 25 per 10,000, in The Netherlands 13 per 10,000, in Portugal 21 per 10,000, in Spain Castilla y Leon 27 per 10,000 and in Spain Basque 55 per 10,000. Analysis of secondary contact cases suggested underestimation of incidence between 2.4% in Spain Castilla y Leon and 32.2% in The Netherlands. There was a trend towards incidence at an earlier age in England and Wales and in The Netherlands compared with Portugal and Spain. Whilst there was little problem in reliably identifying the number of incident cases in the recording networks and relating the non-consulting contact cases to them, the security of the denominator remains a problem where networks are comprised of differing categories of health care provider. It is essential that numerator and denominator information are made available specifically for each category.