 The article discusses the limitations of using randomized controlled trials, RCTs, as the only tool to evaluate non-specific effects, NSEs, of vaccines which can be ethically problematic, time-consuming, and expensive. Instead, triangulation of RCTs and observational studies can build a strong argument for causality by merging multiple lines of evidence with different underlying bias structures. The article examines two examples related to measles vaccine and oral polio vaccine, an expert commentary suggests that results from both RCTs and non-RCT studies should be triangulated to strengthen causal interpretation. This article was authored by Christine Stable-Ben, Ayn Berent Fisca, Andreas Rieckman, and others.