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In places where sanitation and hygiene are suboptimal, oral polio vaccine (OPV) has poor immunogenicity, and intestinal immunity wanes following vaccination. In much of the developed world, inactivated polio vaccine (IPV) has replaced OPV. IPV does not induce an effective mucosal immune response or result in viral stool shedding, as OPV does. However, although IPV doesn't cause vaccine-associated paralytic polio (which occasionally occurs after OPV vaccination), the lack of viral shedding doesn't allow IPV to have as widespread an effect in preventing fecal–oral spread in the community.
Given these facts, researchers hypothesized that sequential OPV followed by IPV might boost immunity in children older than age 12 months by stimulating immu…