Statistical analysis of TBE antibody decrease after vaccination

This study is concerned with the statistical analysis of tick-borne encephalitis (TBE) antibody decrease after vaccination. The objective is to derive conclusions about protection duration of vaccination. TBE is a serious infection affecting the central nervous system and Austria is a high risk region of TBE. Active immunization by vaccination prevents TBE and nowadays 88% of Austria's population is vaccinated. The statistical analysis is based on observational data collected at the Institut für Hygiene, Mikrobiologie und Tropenmedizin des Allgemeinen öffentlichen Krankenhauses der Elisabethinen in Linz from persons who deliberately went for a test of their TBE antibody level. In addition to antibody titer obtained from an ELISA-test the data set contains information on sex, number of booster vaccinations, age and time since last vaccination (covariates). For some people repeated measurements are available. Various parametric regression type models have been fit to investigate the decrease of antibody titer after vaccination. Pure regression models turned out not to be adequate for explaining titer decrease due to considerably heterogeneity between individuals. Individual random effects have to be taken into account. Though the limited nature of the data does not allow estimation of individual subject specific profiles all models indicate that vaccination protection might be considerably longer than the currently recommended protection interval of 5 years. A more detailed analysis could be carried out from a study where at least 3 measurements would be available for each individual