Where F k denotes the k-th risk factor, p 1 is the probability of seropositivity to pathogen 1 and S 2 the serological status to pathogen 2. The coefficients a k (k = 0…K) and b are the coefficients of the logistic regression. The interaction between the two pathogens was tested using a likelihood-ratio test (LRT) testing H0: b = 0 vs H1: b=0. The asymptotic chi-square approximation was used to derive the Pvalue of the test of independence between the two viruses [63] .