Estimation of the variation that can be attributed to different levels in a clinical trial of a vaccine

Ana Belén Garcia Clavero, Simon Bahrndorff, Birthe Hald, M. Madsen, Håkan Vigre

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Abstract

This presentation is focusing on the interaction between the experimental design of a vaccination trial and appropriate data analysis using a trial of a vaccine against Campylobacter in broilers as an example. This study was designed using four rotations with eight isolators per rotation (10 chickens per isolator). Treatment was administered at isolator level on day 14 (vaccine or placebo). The broilers were inoculated with Campylobacter jejuni at day 31 and slaughtered at day 42. The numbers of Campylobacter (cfu/g)
were obtained in the laboratory using selective cultivation methods and log transformed to obtain a Gaussian distribution. Initially, the effect of the vaccine was analyzed using all data in a t-test. Subsequently, the t-test was stratified by rotation. Finally, mixed linear models were used, taking into account the physical hierarchical setup of the trial. Results from the t-test indicate an effect of the vaccine, whereas the result obtained from the complex model indicated high variability between birds and isolators but not significant vaccine effect. The apparent observed differences between vaccinated and placebo groups in the t-tests could be attributed to the variation between incubators. Broilers in the same isolator had more equal numbers of C. jejuni compared to chickens in other incubators. It is possible that chickens in the same isolator re-infect each other with Campylobacter. In this study, the design effect was considerable, reducing the effective sample size (67 animals instead of the 290 animals included). The clustered design used in this trial was trying to emulate the clustering effect found in broiler flocks and farms.
Original languageEnglish
Title of host publicationBook of Abstracts of the 13th International Symposium on Veterinary Epidemiology and Economics
Number of pages1
PublisherWageningen Academic Publishers
Publication date2012
Pages393
Publication statusPublished - 2012
Event13th Conference of the International Society for Veterinary Epidemiology and Economics - Maastricht, Netherlands
Duration: 20 Aug 201224 Aug 2012
Conference number: 13

Conference

Conference13th Conference of the International Society for Veterinary Epidemiology and Economics
Number13
CountryNetherlands
CityMaastricht
Period20/08/201224/08/2012

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