Modelling antibody response to measles vaccine and subsequent waning of immunity in a low exposure population

We analysed measles antibody data from 1141 vaccinated Canadian children ag ed 4-16 years to investigate factors associated with seropositivity and mag nitude of prevailing antibody titres. We used a mixed Bernoulli/log-gamma m odel which takes account of the possibility of primary vaccine failures, of censored observations due to the lower detection limit of antibody assays and of a potentially skewed distribution of the logarithm of titres, While controlling for all significant variables, we observed that measles antibod y titres decline at a mean rate of 5.6% per annum (approximate 95% CI; 3.3- 7.7) equivalent to a half life of 12 years. Further, we noted a detectable linear relationship with respect to age at immunisation such that the mean increase in measles antibody titre of an infant who responds to vaccination gained by delaying vaccination from 12 to Is months of age is 11.1% (appro ximate 95% CI; 4.4-18.3). (C) 2000 Elsevier Science Ltd. All rights reserve d.