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a, b Maintainable population immunity for maternal immunity waning at 6 and 3 months, respectively, in a region with a developing age structure. c, d Maintainable population immunity for maternal immunity waning at 6 and 3 months, respectively, in a region with developed age structure. Strategies that meet the 95 % threshold are shown in black; those that meet the 90 % threshold are shown in gray
