Figure 6: Environmental effects on basic reproductive ratio, 𝑅 0 . (a) Seasonal change in virus inactivation rate, 𝑘 v i r , e n v (Figure 1; (15), (18)), giving virus inactivation by temperature alone. 𝑇 a i r is mean daily air temperature (appropriate to southern Britain) (19); 𝑇 a i r 0 is an assumed temperature threshold for virus inactivation, here equal to 13°C ((18), Table 1)); 𝑅 𝟎 is the seasonally modulated basic reproductive ratio (20), calculated assuming temperature is constant. (b) For different values of mean annual air temperature, 𝑇 a i r , m e a n (19), basic reproductive ratio 𝑅 0 (20) is seasonally modulated as in (a) via the seasonal modulation of 𝑘 v i r , e n v , for a constant threshold temperature for virus inactivation of 𝑇 a i r 0 = 13°C (18), giving for 𝑅 0 a maximum, minimum, annual mean, and amplitude (maximum-minimum). 𝑁 f d y denotes the number of forcing days per year (a “forcing day” is a day on which air temperature is sufficiently high as to increase virus mortality).