An evaluation of the indirect cohort method to estimate the effectiveness of the pneumococcal polysaccharide vaccine

We examined the validity of the indirect cohort method as a rapid assessment tool to estimate pneumococcal polysaccharide vaccine effectiveness (VE). Using evidence from published clinical trials, we reviewed the primary assumption about the appropriateness of the control group underpinning the indirect cohort method, which is that the risk of non-vaccine type invasive pneumococcal disease is equal for vaccinated and unvaccinated participants. We found an absence of evidence to support the non-differential risk assumption
for non-vaccine type invasive pneumococcal disease occurring among clinical trial participants. In those instances where the design has been utilised, we also note that these typically rely on very small numbers of non-vaccine type invasive pneumococcal disease adding to the concerns as an unreliable comparator. We do not consider the indirect cohort method to be a valid tool for rapid assessment of pneumococcal polysaccharide VE.