Morrison VA, Johnson GR, Schmader KE, Levin MJ, Zhang JH, Looney DJ, Betts R, Gelb L, Guatelli JC, Harbecke R, Pachucki C, Keay S, Menzies B, Griffin MR, Kauffman CA, Marques A, Toney J, Boardman K, Su SC, Li X, Chan IS, Parrino J, Annunziato P, Oxman MN. Long-term persistence of zoster vaccine efficacy. Clin Infect Dis. 2015 Mar 15; 60(6):900-9.