Schematic diagram of a simple VZV model. The included compartments contain the susceptible to varicella (S), individuals exposed to varicella (E), those who are infected with varicella (I), who have recovered and are protected from VZV (R), who are weakly immune (W), i.e. individuals partly susceptible to varicella and HZ, individuals infected with zoster (IZ) and those who have recovered from zoster and are immune to VZV (RZ) and individuals who lost immunity after recovery from zoster and are susceptible to VZV (SZ). The arrows indicate movements between compartments at the rates marked by Greek letters. The rates are explained in Table 1.