Methodological issues in design and analysis of a matched case-control study of a vaccine's effectiveness

Objective: Case-control studies of the effectiveness of a vaccine are useful to answer important questions, such as the effectiveness of a vaccine over time, that usually are not addressed by prelicensure clinical trials of the vaccine's efficacy. This report describes methodological issues related to design and analysis that were used to determine the effects of time since vaccination and age at the time of vaccination. Study Design and Setting: A matched case-control study of the effectiveness of varicella vaccine was conducted. Results: Sampling procedures and conditional logistic regression models including interaction terms are described. Conclusion: Use of these methods will allow investigators to assess the effects of a wide range of variables, such as time since vaccination and age at the time of vaccination, on the effectiveness of a vaccine. (C) 2007 Elsevier Inc. All rights reserved.