Mathematical models for assessment of long-term persistence of antibodies after vaccination with two inactivated hepatitis A vaccines

Very few studies with inactivated hepatitis A vaccines were designed for lo ng-term follow-up of antibody persistence. Based on the serological data fr om these vaccine trials, mathematical models were developed to predict the decrease of anti-hepatitis A virus (anti-HAV) antibodies after vaccination. This study was designed to compare Avaxim (0-6 months) to Havrix 720 (0-1- 6 months). In this paper, both groups of vaccinees are described considerin g the age, gender, and weight of the subjects at enrollment. For mathematic al modelling, two different approaches were used: one starting the calculat ions from the geometric mean titres (GMTs) at each point in time, the other basing the calculations on individual anti-HAV titres. Both vaccines are v ery immunogenic, although Avaxim shows a higher GMT at each point in time. When these data are used in mathematical models to predict the persistence of anti-HAV antibodies, both vaccines (Avaxim and Havrix 720) show similar long-term antibody kinetics. Antibody levels greater than or equal to 20 ml U/ml are estimated to last on average for at least 10 years after completio n of the full vaccination course. Ten years after the full course, approxim ately 53% of subjects are estimated to have antibody levels greater than or equal to 20 mlU/ml. At 15 years, these levels will be maintained by about 34% of vaccinees. Avaxim and Havrix 720 show a similar long-term profile of persistence of anti-HAV. A mathematical model based on GMTs appeared to gi ve equivalent results to a model based on individual serological data. The GMT method is easier to apply than the individual based method. However, th e advantage of the latter method is the possibility of calculating confiden ce limits for the predicted values and making estimates of the percentage o f subjects having a certain level of antibody titres at a certain time. J. Med. Virol. 60:1-7, 2000. (C) 2000 Wiley-Liss, Inc.