Vaccine Efficacy

The objective of this study was to determine the effect of the porcine circovirus type 2 (PCV2)
vaccination on the levels of viremia, the number of viremic-positive pigs, and production
performance [i.e. nursery mortality, post-weaning mortality, and average daily weight
gain (ADWG)] under field conditions. There were 140 farrow-to-finish pig herds involved in
this study. The vaccination of piglets was implemented in 82 of the 140 herds. In each herd
blood samples were collected from sows and pigs in different age category. In addition, a
questionnaire regarding the production performance was provided for each herd. Results
demonstrate that the vaccination of piglets prevented the development of viremia in 23.2% of
herds. Significant decreases in the levels of PCV2 DNA in serum and in the number of viremic
pigs were also noted. These results indicate that the vaccination of piglets against PCV2 is a
useful tool in controlling the PCV2 infection in herds with a high risk of a wide range of viral
and bacterial agents, poor management strategies, and a low level of biosecurity practices.

To fully understand the assessments of vaccine efficacy and safety, the crucial information needed for regulatory approval, one must understand the principles of vaccine epidemiology. In this review article, we go through some of the key concepts in vaccine epidemiology.

We present a method to estimate the time-dependent vaccine efficacy from the cohort-specific vaccination coverage and from data on the vaccination status of cases and apply it to a measles epidemic in Germany which involved 529 cases, 88 of whom were vaccinated and 370 unvaccinated (for the remaining 71 cases the vaccination status is unknown). Our epidemiological model takes into account that maternal antibodies prevent successful vaccination and that vaccine immunity may be lost over time. Model parameters are estimated from the data using maximum likelihood. Vaccination coverage, as determined in school surveys, ranged from 27.6 per cent for the cohort born in 1974 to 85 per cent for the 1986 cohort, which is far too low to prevent measles transmission. Cohorts for which no school surveys were performed are omitted from analysis. Thus, sufficient data are available for only 282 cases, 69 of whom are vaccinated. According to our estimates, measles vaccinations provided no immunity before 1978 (95 per cent CI: 0 to 47 per cent), for the period 1978-1982, the estimated vaccine efficacy was 80per cent (95 per cent CI: 67 to 89 per cent), and for 1982–1990 it was 97 per cent (95 per cent CI: 93 to 99 per cent). After 1990, the estimated value dropped to 89 per cent, but its confidence interval widely overlaps with that of the previous period (95 per cent CI: 74 to 97 per cent). Loss of immunity was estimated to be zero (95 per cent CI: 0 to 0.003/year). Several sensitivity analyses were performed with respect to the model assumptions. A modified model which assumed constant efficacy at all vaccination times yielded a high estimate of 96 per cent (95 per cent CI: 92 to 98 per cent) for primary vaccine efficacy but also a high loss rate of immunity of 0.007/year (95 per cent CI: 0.001 to 0.012) to explain the high fraction of vaccinated cases among older individuals. The likelihood score value is however significantly inferior compared to the score value of the model with time-dependent vaccine efficacy. Copyright © 2002 John Wiley & Sons, Ltd.