Passively immune infants
S Susceptibles
E Exposed people in the latent period
I Infectives
R Recovered people with immunity
m, s, e, i, r Fractions of the population in the classes above
β Contact rate
1/δ Average period of passive immunity
1/ε Average latent period
1/γ Average infectious period
R0 Basic reproduction number
σ Contact number
R Replacement numbe