Description of selected epidemiological risk factor studies.

VZV varicella-zoster virus; HZ herpes zoster; CP chickenpox; HH household; OR odds ratio; aOR adjusted odds ratio; NS not significant; PY person-years.*H = High: the quality of methods used in this paper permits the results, within the scope of the study design, to be interpreted with at the most a few remarks. M = Medium: the quality of methods used in this paper permits the results, within the scope of the study design, to be interpreted, but with some caution. L = Low: the quality of methods used in this paper urges the reader to interpret the results, even within the scope of the study design, with sufficient caution.£See Table S2.**The ‘B’ statement expresses whether the study supported the existence of exogenous boosting (‘+) or not (‘−’).