Polio eradication: next steps and future challenges

摘要

In 1988, the World Health Assembly resolved to eradicate polio. At that time, polio was endemic in 125 countries and paralysed around 1,000 children per day. Since then, polio cases due to infection with wild poliovirus (WPV) have decreased by more than 99.9% from over 350,000 cases a year to 37 cases in 2016 and 22 in 2017 [ 1 ]. Of the three WPV serotypes, 1, 2 and 3, WPV2 has not been detected since 1999 and was declared eradicated in September 2015. This allowed a global switch from live trivalent oral polio vaccine (tOPV) to live bivalent oral polio vaccine (bOPV), eliminating the need for live type 2 poliovirus vaccine strains. At the same time, this switch has created a need for universal use of inactivated polio vaccine (IPV) to ensure immune protection against type 2 poliovirus. Vigorously applied disease control programmes have clearly made huge contributions to the goal of eradication of poliomyelitis, and we are tantalisingly close to the complete elimination. Clinical and virological surveillance using the acute flaccid paralysis (AFP) case definition, in tandem with comprehensive vaccination programmes using OPV have been extremely successful in high disease burden countries. WPV3 was last detected in November 2012 in Nigeria, and since this time WPV1 has been the sole circulating WPV type globally. WPV transmission has persisted in only two countries: Afghanistan and Pakistan, although in August 2016 it was also detected in Nigeria [ 1 ]. As we begin to see the light at the end of the tunnel, a relentless focus on achieving complete vaccination coverage in the areas still using OPV, together with a global commitment to universal IPV coverage and diverse approaches to surveillance, are needed to achieve the final target. Endgame strategy The Polio Eradication and Endgame Strategic Plan (2013–18) set the goal of a polio-free world by the end of 2018 [ 2 ]; this was recently extended until at least 2021 [ 3 ]. Achieving this challenging goal requires: (i) completion of WPV eradication to eliminate the risk of WPV transmission; (ii) cessation of the use of oral polio vaccine (OPV) after eradication completion to eliminate the risks of chronic immunodeficiency-associated vaccine-derived poliovirus (iVDPV) cases and circulating vaccine-derived poliovirus (cVDPV) causing outbreaks due to person-to-person transmission in areas with poor vaccination coverage [ 4 , 5 ]; and (iii) implementation of poliovirus safe-handling and containment measures following the World Health Organization (WHO) Global Action Plan (GAPIII) to minimise poliovirus facility-associated risk after type-specific eradication of wild polioviruses and sequential cessation of oral polio vaccine use to minimise the risks of reintroduction of virus into the polio-free community [ 6 ]. The last pockets of WPV circulation are in Afghanistan and Pakistan, where the physical challenges of landscape and isolated communities are compounded by a challenging socio-political environment that affects the timely delivery of vaccines [ 7 ]. The commitment of local health authorities is slowly but gradually leading to improvements in vaccine coverage [ 8 ]. Delivery of vaccine programmes has never been more essential. Clinical surveillance The assessment of polio elimination status in a country is based upon demonstration of routinely high uptake of vaccine in children and evidence of strong polio surveillance. One of the hallmarks of the smallpox elimination campaign in its final stages in the 1970s was relentless tracking and detailed investigations of possible cases, however difficult the circumstance or how improbable the clinical case. Smallpox had a distinct clinical presentation making it easier to recognise compared with polio, where the key indicator clinical syndrome for the elimination is acute flaccid paralysis (AFP). This occurs in less than 5% of poliovirus-infected individuals, and is also the result of poorly understood aetiologies such as Guillain-Barré syndrome. Currently, many countries struggle to undertake AFP surveillance. Polio-associated AFP is a rare disease overall, and its declining incidence and lack of perceived importance has led to difficulties in use and verification of individual cases [ 3 ]. As discussed in this week’s Eurosurveillance report of the Spanish experience of AFP surveillance over the past 20 years, in at least a third to two thirds of cases the supporting virological investigations may also be less than optimum [ 9 ]. The findings emphasise that the overall sensitivity of passive AFP case finding, as a tool for detection of polio circulation in the era of eradication, is insufficient and needs to be supplemented. Awareness raising within the clinical and paediatric communities, of the importance of timely notification of possible AFP cases and detailed and disciplined investigation, is necessary to overcome the presumption that polio has indeed disappeared and to dispel the notion that case follow-up