Hopf curves for selected parameter pairs: (a)
β is a varicella transmission coefficient and ζ is a varicella force of infection reduction coefficient for those who become reinfected following clearance of varicella; (b)
σ is a loss of full immunity rate and δ is a rate of HZ reactivation after loss of immunity following recovery from HZ; (c)
θ is a rate of loss of immunity to VZV after recovery from HZ and κ is a boosting coefficient; (d)
α is a varicella force of infection reduction coefficient for those who become reinfected following HZ attack and η is a HZ reactivation rate. Shaded areas limited by these curves correspond to periodic solutions of (1).