Effects of recruitment strategies and demographic factors on inclusion in a large scale vaccination trial in adults 65 years and older

Large-scale randomized studies generate the highest level evidence for medical interventions. Yet, successful recruitment frequently is challenging, especially when targeting elderly populations. Although several studies investigated specific recruitment barriers, there is little quantitative understanding of such barriers. We therefore determined associations between patient related and study-related factors and study inclusion in healthy elderly (>65 years) invited to participate in a double-blind placebo-controlled randomized study to determine effectiveness and safety of a 13-valent pneumococcal vaccine for community-acquired pneumonia in the Netherlands. Inclusions for this study took place between September 2008 and January 2010. The analysis was performed on replies to invitations sent between February 2009 and October 2009. In our analyses 260,700 replies from this period resulted in 48,982 candidates included in the study (18.8%). Study inclusion was associated with travel time to the vaccination site (decline of 0.6% per 4 min travel time, adjusted odds ratio (OR) 0.972, 95% CI 0.964-0.980), number of published advertorials in local newspapers (increase of 0.4% per consecutively placed advertorial, adjusted OR 1.030, 95% CI 1.026-1.035), age (decline of 0.7% per year, adjusted OR 0.953, 95% CI 0.951-0.955) and male gender (adjusted OR 0.588 versus female, 95% CI 0.576-0.599). Introduction letters sent on behalf of general practitioners prior to the actual invitation letter were not associated with study inclusion. Careful consideration of these parameters in study preparation may facilitate more successful patient recruitment in clinical trials in healthy elderly.