Vaccine-derived polio in Nigeria

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Polio vaccine comes in two types, the original inactivated Salk vaccine (IPV) and the somewhat later attenuated Sabin vaccine (OPV) [*]. The Sabin vaccine can be delivered orally, provides superior immunity, and because it's live, it can infect people other than the vaccinated, providing some immunity to them. The disadvantage is that it can occasionally convert in the body into an infectious form. That's what's been happened in Nigeria:
So far, there are 69 confirmed cases of paralysis, and more suspected, caused by VDPV in nine northern Nigeria states, says Kew. The case count seems certain to rise. About half the cases have occurred around Kano, a largely Muslim state where anti-Western sentiment and rumors that the vaccine caused sterility or AIDS led several states to halt polio vaccination in 2003. After repeated demonstrations of the vaccine's safety and considerable behind-the-scenes diplomacy, vaccinations resumed about a year later, but the damage had already been done.


The current outbreak came to light when a technician at the CDC polio lab noticed a preponderance of type 2 virus in the isolates sent in from northern Nigeria. That instantly raised suspicion, Kew says, because wild type 2 poliovirus has been eradicated globally. That meant the only possible source was the trivalent vaccine, which had been used in Nigeria in preboycott campaigns. Since Nigeria resumed vaccinations in 2004, says Kew, it had "quite properly" been using the more effective monovalent vaccines against wild types 1 and 3 in its campaigns. Genetic analysis quickly confirmed the source; it also suggests that several VDPVs emerged independently in 2005 and 2006, multiple times.

The problem here seems to be that if your overall vaccination rate is really low, then any cases of VPDV can spread through the rest of the population:

In earlier outbreaks, circulating VDPVs have been relatively easy to stamp out, but this one has persisted despite four campaigns with trivalent OPV in the past year. "We suspect it is simply because the coverage was not adequate; we don't believe there is anything exceptional about this virus," says Kew. As evidence, he notes that two VDPV strains jumped from Nigeria to Niger, where routine vaccination is almost 90%. Both "barely made it 5 kilometers before they dead-ended," he says.

Unlike Nigeria, the vaccine of choice in the US is IPV.


Also, the OPV vaccine, IIRC, provides better herd immunity. For some reason I thought that with IPV you can be immune but can still be a carrier.

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