Surveillance for rare infectious diseases: is one passive data source enough for Haemophilus influenzae?

The completeness of a compulsory reporting system of systemic Haemophilus influenzae infections in children in Germany is studied by means of cross-linking registry data from three sources and applying capture-recapture methods.

Cases were collected for the years 2001-05 by three national data sources: a passive administration registry (SurvNet@RKI), an active hospital surveillance system and an active laboratory surveillance system. The case definition required cultural detection of H. influenzae in blood or cerebrospinal fluid. Linkage was carried out by month and year of birth, sex, geographical region and date of disease onset. Capture-recapture models were used to estimate the incidence of invasive H. influenzae infections.

SurvNet@RKI reported 113 H. influenzae and 38 H. influenzae type b (Hib) cases, compared to a total of 231 and 68 cases, respectively, reported by all three sources combined. Best-fitting 3-source capture-recapture estimations amounts to 258 (95% confidence interval: 247-276) H. influenzae and 71 (69-74) Hib cases. SurvNet@RKI data depicted a similar decrease in annual H. influenzae cases as the capture-recapture estimates but failed to detect the underlying decrease in Hib cases which was observed in the capture-recapture estimates due to a considerable annual variability of ascertainment of serotyped cases in SurvNet@RKI ranging from 14% to 69%.

Because of small variability of ascertainment, the compulsory passive reporting system depicted trends in H. influenzae incidence, although less than half of the cases were ascertained. However, time trend in Hib cases could not be depicted, because of highly variable serotyping proportions.