the wild virus, but where vaccination rates have fallen. oral vaccines can also give rise to persistent infection in immunodeficient individuals, with the virus eventually mutating into a more virulent immunodeficiency-associated vaccine-derived poliovirus (ivdpv). in particular, the type 2 strain seems prone to reversions, so in 2016 the eradication effort abandoned the trivalent oral vaccine containing attenuated strains of all three virus types and replaced it with a bivalent oral vaccine lacking the type 2 virus, while a separate monovalent type 2 vaccine (mopv2) was to be used only to target existing cvdpv2 outbreaks. further, a novel oral vaccine targeting type 2 (nopv2) that has been genetically stabilized to make it less prone to give rise to circulating vaccine-derived strains is now in limited use. eradication efforts will eventually require all oral vaccination to be discontinued in favor of the use of injectable vaccines. these vaccines are more expensive and more difficult to deliver, and they lack the ability to induce contact immunity because they contain only killed virus, but they likewise are incapable of giving rise to vaccine-derived viral strains. surveillance a global program of surveillance for the presence of polio and the poliovirus plays a critical role in assessment of eradication and in outbreak detection and response. two distinct methods are used in tandem: acute flaccid paralysis (afp) surveillance and environmental surveillance. monitoring for afp aims at identifying outbreaks of polio by screening patients displaying symptoms consistent with, but not exclusive to, severe poliovirus infection. stool samples are collected from children presenting with afp and evaluated for the presence of poliovirus by accredited laboratories in the global polio laboratory network. since rates of non-polio afp are expected to be constant and large compared to the number of polio cases, the frequency of non-polio afp reported in a population is indicative of the effectiveness of surveillance, as is the proportion of afp patients from whom high-quality stool samples are collected and tested, with a target of at least 80%. environmental surveillance is used to supplement afp surveillance. this entails the routine testing of sewage samples for the presence of virus, which not only allows the effectiveness of vaccination efforts to be evaluated in countries with active transmission, but also allows the detection of new outbreaks in countries without known transmission. in 2018, the gpei conducted environmental surveillance in 44 countries, 24 of which are in africa. obstacles among the greatest obstacles to global polio eradication are the lack of basic health infrastructure, which limits vaccine distribution and delivery, the crippling effects of civil war and internal strife, and the sometimes oppositional stance that marginalized communities take against what is perceived as a potentially hostile intervention by outsiders. another challenge has been maintaining the potency of live (attenuated) vaccines in extremely hot or remote areas. the oral polio vaccine must be kept at 2 to 8 °c (36 to 46 °f) for vaccination to be successful. an independent evaluation of obstacles to polio eradication requested by the who and conducted in 2009 considered the major obstacles in detail by country. in afghanistan and pakistan, the researchers concluded that the most significant barrier was insecurity, but that managing human resources, political pressures, the movement of large populations between and within both countries, and inadequately resourced health facilities also posed problems, as did technical issues with the vaccine. in india, the major challenge appeared to be the high efficiency of transmission within the populations of bihar and uttar pradesh states, set against the low (~80% after three doses against type 1) seroconversion response seen from the vaccine. in nigeria, the most critical barriers identified were management issues, in particular the highly variable importance ascribed to polio by different authorities at the local government level, although funding issues, community perceptions of vaccine safety, inadequate mobilisation of community groups, and issues with the cold chain also played a role. in those countries where international spread from endemic countries had resulted in the reestablishment of transmission, namely angola, chad, and south sudan, the key issues identified were underdeveloped health systems and low routine vaccine coverage, although the low level of resources committed to angola and south sudan for the purpose of curtailing the spread of polio and climatic factors was also identified as playing a role. two additional challenges are found in unobserved polio transmission and in vaccine-derived poliovirus. first, most individuals infected with poliovirus are asymptomatic or exhibit minor symptoms, with fewer than 1% of infections leading to paralysis, and most infected people are unaware that they carry the disease, allowing polio to spread widely before cases are seen. in 2000, using new screening techniques for the molecular characterization of outbreak viral strains, it was discovered that some of the outbreaks were actually caused by circulating vaccine-derived poliovirus, following mutations or recombinations in the attenuated strain used for the oral polio vaccine. this discovery altered the strategy for the discontinuation of vaccination following polio eradication, necessitating an eventual switch to the more expensive and logistically more problematic inactivated polio vaccine, as continued use of the oral inactivated virus would continue to produce such revertant infection-causing strains. the risk of vaccine-derived polio will persist long after the switch to inactivated vaccine, as a small number of chronic excretors continue to produce active virus for years (or even decades) after their initial exposure to the oral vaccine. in a 2012 interview with pakistani newspaper dawn, dr. hussain a. gezari, the who's special envoy on global polio eradication and primary healthcare, gave his views on obstacles to eradication. he said that the biggest hurdle preventing pakistan from becoming polio-free was holding district health officials properly accountable—in national eradication campaigns officials had hired their own relatives, even young children. gezari asked, "how do you expect a seven-year-old thumb-sucking kid to implement a polio campaign of the government?" and added that, in spite of this, "the first national campaign was initiated by your government in 1994 and that year pakistan reported 25,000 polio cases, and the number was just 198 last year, which clearly shows that the programme is working." opposition to vaccination efforts