Varicella-zoster virus: pathogenesis, incidence patterns and vaccination programs.

Varicella or chickenpox is a common and highly contagious exanthematic disease caused by the varicella-zoster virus (VZV) that during primary infection has the ability to establish latency. VZV reactivation, even decades after primary infection, causes herpes zoster. In healthy immunocompetent subjects, children in particular, varicella results in mild to moderate illness and for this reason, regardless its high morbidity, it is not considered a public health priority. Varicella still represents the most widespread vaccine preventable childhood infectious disease in industrialized countries; due to its relevant burden on healthcare resources several countries has introduced varicella vaccination into the recommended routine childhood national immunization schedule. Nowadays, live attenuated monovalent and combined MMRV vaccines are licensed worldwide. The use of several millions of doses has demonstrated the excellent safety and efficacy level of varicella vaccines as well as of combined MMRV vaccines. Universal vaccination adopted in many countries with a two-dose strategy has allowed to significantly reducing morbidity and mortality of this infectious disease. Anyway, an ample international debate is ongoing on the time range to be used between the two doses, and on the safety issues related to the use as first-dose of MMRV vaccine. Taking into account the availability of a zoster vaccine in subjects older than 50 years of age, it will be relevant to clarify if an impact on exogenous boosters and on the epidemiology of herpes zoster can occur after the adoption of extensive varicella immunization.