One-hit models for virus inactivation studies

All biologicals whose production involves materials of human or animal origin are at risk of viral contamination. Testing the capacity of the production processes to remove or inactivate viruses is an essential step in establishing the safety of biological products. The one-hit model which is essentially based on the assumption that the assay will show a positive reaction if and only if there is at least one infectious particle in a small sample drawn from the material, is often used as a basis for the estimation of the number of infectious particles per unit volume, or equivalently, to estimate the ID50 (the dose which results in 50% positive reactions). Due to the availability of computers it is no longer necessary to use inadequate and biased methods like Spearman-Kärber to estimate the ID50. Depending on the details of the experiment the average bias of Spearman Karber ID50 estimates is 10-30%. Maximum likelihood estimation procedures of the parameters, the computation of ID50, reduction factors, and their confidence limits are presented. Furthermore, hints for the design of the experiments are given. The incorporation of kinetics models is also discussed. Although the method represents the state of the art in the biostatistical literature, the problem of random variations of doses has not been addressed appropriately. Based on 36 000 simulated experiments it is shown that the parameters of the model are robust with respect to random variation of doses. Designs using 10-fold dilution series, however, are generally less appropriate and also more affected by dose variability.