The final stages of the global eradication of poliomyelitis

Abstract

The global incidence of poliomyelitis has dropped by more than 99 per cent since the governments of the world committed to eradication in 1988. One of the three serotypes of wild poliovirus has been eradicated and the remaining two serotypes are limited to just a small number of endemic regions. However, the Global Polio Eradication Initiative (GPEI) has faced a number of challenges in eradicating the last 1 per cent of wild-virus transmission. The polio endgame has also been complicated by the recognition that vaccination with the oral poliovirus vaccine (OPV) must eventually cease because of the risk of outbreaks of vaccine-derived polioviruses. I describe the major challenges to wild poliovirus eradication, focusing on the poor immunogenicity of OPV in lower-income countries, the inherent limitations to the sensitivity and specificity of surveillance, the international spread of poliovirus and resulting outbreaks, and the potential significance of waning intestinal immunity induced by OPV. I then focus on the challenges to eradicating all polioviruses, the problem of vaccine-derived polioviruses and the risk of wild-type or vaccine-derived poliovirus re-emergence after the cessation of oral vaccination. I document the role of research in the GPEI's response to these challenges and ultimately the feasibility of achieving a world without poliomyelitis.

Keywords: eradication; polio; poliomyelitis; poliovirus; public health; vaccine.

Figure 1.

The geographical distribution of children with poliomyelitis associated with wild-type poliovirus shown by serotype for 2012 and (inset) the total number of children with poliomyelitis globally by serotype reported each year during 2001–2012. Poliomyelitis as a result of vaccine-derived polioviruses is not shown. India was declared ‘polio-free’ in 2012, the last case reported from West Bengal in January 2011. The arrows above the inset graph indicate when monovalent and bivalent OPVs were first used by the GPEI. Map and data are from WHO (www.polioeradication.org).

Figure 2.

Reasons for missed children during SIA in the first half of 2012 based on independent monitoring data from the three regions that have yet to interrupt indigenous poliovirus transmission: southern Afghanistan, Pakistan and northern Nigeria. The proportion of all children 0–4-years old who were identified as missed is shown by the figures in brackets. In southern Afghanistan the ‘other’ category specifically refers to cases where the child was a neonate, asleep or sick. Southern Afghanistan includes Kandahar, Helmand, Urozgan, Zabul and Nimroz provinces. Northern Nigeria includes Bauchi, Borno, FCT Abuja, Gombe, Jigawa, Kaduna, Kano, Katsina, Kebbi, Plateau, Sokoto, Yobe and Zamfara states. Data courtesy UNICEF PolioInfo (www.polioinfo.org).

Figure 3.

International spread of (a) serotype 1 and (b) serotype 3 wild polioviruses resulting in cases during 2009–2011 based on genetic sequencing information. The arrows indicate the direction of wild-type poliovirus spread and the circles are drawn in proportion to the number of cases that resulted from the importation of virus. Arrows and circles are colour-coded according to the original endemic country source of the virus. Endemic countries during 2009–2011 are shown in grey. Thicker arrows indicate more than one importation during the period of the analysis. At least 83 importations of wild-type poliovirus were detected during this period but many more such events would have occurred without detection of symptomatic cases. The origin and destination of the arrows point towards the centre of each country rather than the regions with circulation except in the case of China and Russia. Plot based on data presented in Kew et al. [135].