Travel vaccines are strongly associated to reduced mortality in prostate cancer patients - a real effect or residual confounding?

Repurposing of existing drugs and vaccines for diseases that they were not originally intended for is a promising research field. Recently there has been evidence that oral cholera vaccine might be used in the treatment of inflammatory disease and some common cancers. Specifically, Ji et al showed that the administration of cholera vaccine after a prostate cancer diagnosis reduced prostate cancer specific mortality rates by almost 50%. In a cohort of men from Stockholm, Sweden, with more detailed cancer data and a higher coverage of exposure to vaccine, we replicated these findings using a marginal structural Cox model. We showed that administration of cholera vaccine after prostate cancer diagnosis is associated with a significant reduction in mortality (HR 0.46, 95% CI 0.31-0.69, p-value 0.0001) even after adjusting for all known confounders. However, the same effect (or even stronger) could be seen for several other traveling vaccines and malaria prophylaxis. Therefore, we conclude that this effect is most likely due to a healthy traveler bias and is an example of residual confounding.