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. 2011 Dec 2;6(12):e28435. doi: 10.1371/journal.pone.0028435

Figure 2. Assessment of bias when estimating VE with the indirect cohort design when there is complete serotype replacement.

Figure 2

VEBroome is observed vaccine effectiveness by the indirect cohort method, VEc is true vaccine effectiveness against carriage, VE is true vaccine effectiveness (combining effectiveness against carriage and IPD given carriage) and Pu is the proportion of carriage that is VT in the unvaccinated. The formula relating these quantities is VEBroome  =  1 – (1-VE)/ (1 + VEc Pu / (1-Pu)). Panel A) VE = 90%, panel B) VE = 70%.