Table 4 Four sub-models from the best model. The symbols “−” and “√” mean the absence and presence of interactions, respectively. Model 6 (i) no interactions between HMPV and RSV (ϕHMPV = ϕRSV = 0); Model 6 (ii) influence of HMPV on RSV (ϕRSV ≠ 0, ϕHMPV = 0), Model 6 (iii) influence of RSV on HMPV (ϕRSV = 0, ϕHMPV ≠ 0) and Model 6 (iv) interactions between HMPV and RSV (ϕHMPV ≠ ϕRSV ≠ 0)