Eradication of Poliovirus: Fighting Fire With Fire

(See the article by Wassilak et al., on pages 898–909.) Endemic wild polioviruses have been eliminated from most of the world, and the number of human paralytic cases has been reduced by >99%, from an estimated annual incidence of >500,000 cases to 15 outbreaks of paralytic poliomyelitis caused by circulating VDPV (cVDPV) have been recognized throughout the world 1. Such outbreaks have shared one epidemiological characteristic. They have occurred in areas where OPV vaccination coverage has been incomplete; thus, >50% of children remained susceptible. Under these circumstances, cVDPV can circulate for many generations, infect large numbers of persons, and cause outbreaks of paralytic poliomyelitis. The article by Wassilak et al 15 in this issue of the Journal and a companion article 16 describe the most significant of these outbreaks of cVDPV. The Nigerian epidemic, in which type 2 VDPV has caused >300 paralytic cases, began in 2005 and has continued through 2010. Because wild-type 2 poliovirus causes only 1 paralytic case per 2000 infections 1, the Nigerian outbreak might represent >600,000 infections with virulent VDPV. Under what circumstances did this outbreak occur? First, similar to other outbreaks of cVDPV, the epidemic was concentrated in the northern region of Nigeria, where there were relatively low rates of OPV vaccination 15. Second, during 2006–2010, most of the vaccination campaigns in Nigeria used either monovalent or, more recently, bivalent vaccine lacking type 2 OPV. The decision to use these formulations was based on an attempt to control wild-type 1 and 3 polioviruses at a time when wild-type 2 had been eliminated. Monovalent and bivalent formulations that omit type 2 OPV are much more effective than is trivalent OPV 17, 18. In 2009, the dramatic increase in cases due to type 2 cVDPV led to several rounds of trivalent OPV, which may account for the rapid decrease in the number of type 2 cases in 2010. However, this outbreak has not yet been terminated, posing the potential threat of re-introduction of virulent type 2 polioviruses. Because wild-type 1 and 3 polioviruses frequently spread from Nigeria to neighboring countries in Africa, this constitutes a significant contingency 19. The occurrence of repeated outbreaks of cVDPV and the magnitude of the Nigerian epidemic have sent a clear message. True eradication can only be achieved with the elimination of all circulating polioviruses. In countries or scontinents where wild polioviruses have been eliminated, there should be a transition from OPV to inactivated poliovirus vaccine. Many industrialized countries have already made this shift, which occurred in the United States during 1998–2000 14. Although there is no universal consensus, a number of experts have advocated this strategy 20–27. Because inactivated poliovirus vaccine is expensive to manufacture and must be injected, this approach is costly. Several donors (including Rotary International and the Bill and Melinda Gates Foundation) have made significant commitments to underwrite this campaign for low-income countries. Although recent history compels caution, it appears that the world may be on the cusp of elimination of indigenous wild polioviruses. When this goal is achieved, it will then be necessary to terminate the use of OPV if true eradication of circulating polioviruses is to be accomplished.