Natural immunity against capsular group X N. meningitidis following an outbreak in Togo, 2007 by Norheim, G, Mueller, JE, Njanpop-Lafourcade, B-M, Delrieu, I, Findlow, H, Borrow, R, Xie, O, Nagaputra, J, Ramasamy, R, Dold, C, Tamekloe, TA, Rollier, CS, Watt, H, Kere, AB, Næss, LM, Pollard, AJ