Evaluation of the Indirect Effects of a Pneumococcal Vaccine in a Community-Randomized Study

Abstract

When a sufficiently high proportion of a population is immunized with a vaccine, reduction in secondary transmission of disease can confer significant protection to unimmunized population members. We propose a straightforward method to estimate the degree of this indirect effect of vaccination in the context of a community-randomized vaccine trial. A conditional logistic regression model that accounts for within-randomization unit correlation over time is described, which models risk of disease as a function of community-level covariates. The approach is applied to an example data set from a pneumococcal conjugate vaccine study, with study arm and immunization levels forming the covariates of interest for the investigation of indirect effects.

Key Words:

• Community-randomized trial
• Indirect effects
• Pneumococcal vaccine

ACKNOWLEDGMENT

This work has been supported in part by grants from Wyeth Lederle Vaccines and USAID (HRN-A-00-96-9006). The opinions of the authors are not necessarily those of the Indian Health Service.

Notes

* Reference category: Units that received MnCC vaccine which on a given day had less than 25% of children enrolled in the study. The CMLEs are the log rate ratios comparing incidence in non-enrolled children in the given category to the reference category.