Figure 2From: Age groups and spread of influenza: implications for vaccination strategy Graphical illustration of the basic reproduction number for n = 2. The two blue arrows denotes within-group infections (R 11 and R 22) and the black arrows denote the inter-group infection cycle (R 12 R 21). The term R 11 R 22 subtracted in (3) accounts the redundancy due to adding both R 11 and R 22.Back to article page