Epidemiology and Infection

Original Papers

Estimating the impact of vaccination using age–time-dependent incidence rates of hepatitis B

N. HENSa1 c1, M. AERTSa1, Z. SHKEDYa1, P. KUNG'U KIMANIa2, M. KOJOUHOROVAa3, P. VAN DAMMEa4 and Ph. BEUTELSa4

a1 Center for Statistics, Hasselt University, Diepenbeek, Belgium

a2 Kenya Institute of Medical Research, Nairobi, Kenya

a3 National Center of Infectious and Parasitic diseases, Department of Epidemiology, Sofia, Bulgaria

a4 Centre for the Evaluation of Vaccination, Epidemiology and Community Medicine, University of Antwerp, Antwerp, Belgium

Abstract

The objective of this study was to model the age–time-dependent incidence of hepatitis B while estimating the impact of vaccination. While stochastic models/time-series have been used before to model hepatitis B cases in the absence of knowledge on the number of susceptibles, this paper proposed using a method that fits into the generalized additive model framework. Generalized additive models with penalized regression splines are used to exploit the underlying continuity of both age and time in a flexible non-parametric way. Based on a unique case notification dataset, we have shown that the implemented immunization programme in Bulgaria resulted in a significant decrease in incidence for infants in their first year of life with 82% (79–84%). Moreover, we have shown that conditional on an assumed baseline susceptibility percentage, a smooth force-of-infection profile can be obtained from which two local maxima were observed at ages 9 and 24 years.

(Accepted April 12 2007)

(Online publication May 17 2007)

Correspondence:

c1 Author for correspondence: Dr N. Hens, Center for Statistics, Hasselt University, Campus Diepenbeek, Agoralaan 1, 3590 Diepenbeek, Belgium. (Email: niel.hens@uhasselt.be)

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