Surveillance of measles - A comparison of active and passive methods

Background: The Catalan Health Plan includes the elimination of indigenous measles as one of its objectives for the year 2000. Together with an adequa te vaccination policy, epidemiological surveillance is a critical component in this strategy. The objective of the study is to compare the utility of active and passive surveillance methods in detecting and characterising cas es of measles. Methods: Active surveillance was carried out by epidemiologi sts who revised hospital discharges and questioned the staff in health cent res about cases and passive surveillance was carried out by receiving physi cian's notification of cases. The study was done throughout the whole terri tory of Catalonia in 1996. For each case, laboratory results, age, probable site of transmission, size of the region of origin and immunisation status were investigated. Results: One hundred and seventy-one cases were detecte d, of which 96 (56.1%) were confirmed and 52 (30.4%) laboratory confirmed. The positive predictive values for passive and active surveillance were 54. 6% (95% CI: 46.4-63.6) and 60.9% (95% CI: 44.5-75.8) respectively. The prop ortion of patients under 15 years was higher in cases detected by passive s urveillance than in those detected by active surveillance (OR = 2.9 and 95% CI: 1.3-7.0). The probable site of transmission was less frequently known by passive surveillance than by active surveillance (OR = 0.2 and 95% CI: 0 .07-0.4). No association was found between the method of surveillance and t he size of the region or immunisation status. Conclusion: A reasonable stra tegy for measles surveillance appears to be the individualised notification of the disease by physicians followed by surveillance activities directed at confirming the case and identifying its origin.