By Jaime Sepulveda, MD, MPH, MSc, DrSc
In 1988, all member countries of the World Health Organization committed to eradicating the infectious disease poliomyelitis (polio) by the year 2000. The results of this programme are impressive: an estimated 350,000 cases of paralysis-causing polio in 1988 has been reduced to just 8 caused by wild poliovirus this year (as of 12 July), according to the Global Polio Eradication Initiative (go.nature.com/2thuu4d). Successful eradication is very close, but polio is still endemic in Pakistan, Afghanistan and Nigeria. Why? The answer, plain and simple, is that eliminating the transmission of poliovirus in places that are affected by armed conflict and have poor sanitation is more complex than doing so in richer societies that have adequate public health systems. Writing in The Lancet Global Health, Habib et al.1 report the results of a well-designed clinical trial in Pakistan that compared different approaches for the delivery of a polio vaccination programme in three areas where endemic poliovirus transmission and high insecurity coexist.
First, some context. There are two main types of vaccine for preventing polio: an inactivated poliovirus vaccine (IPV) that is injected, and an oral poliovirus vaccine (OPV) that contains live but attenuated strains of the virus. Because of its low cost and ease of administration, OPV has been the main vaccine used in the worldwide eradication programme. But OPV also has disadvantages. In rare circumstances, the attenuated strains used in OPV can mutate and regain both the neurovirulence and transmissibility properties of wild poliovirus2, leading to outbreaks of polio in under-immunized populations. A few outbreaks of such circulating vaccine-derived poliovirus (cVDPV) have been reported in Syria and the Democratic Republic of the Congo this year.
Historically, the Western Hemisphere led the way in polio elimination. Beginning in 1962, Cuba used mass-immunization campaigns involving OPV to become the first country in the Americas to eliminate polio. Countries in Latin America and the Caribbean agreed to implement OPV mass campaigns through national immunization days in 1985. During the civil war in El Salvador in the 1980s, a ceasefire was declared to allow OPV campaigns to take place during 'Days of Tranquillity'. In Mexico, OPV campaigns that initially stood alone eventually integrated other cost-effective health interventions, including the provision of vitamin A, oral rehydration therapy and health education, which were delivered in both homes and local communities3. The last case of polio in the Americas occurred in 1991 in Peru. The many lessons learnt from polio elimination in this region have since been used successfully in other areas of the world4.
In Pakistan, a relatively low proportion of the population is routinely immunized with OPV (the national average is 72%, with a regional variance of 29–90%)5. Habib and colleagues set out to investigate whether integrated approaches could improve such coverage in areas of civil unrest in that country. The authors' community-based study recruited almost 90,000 children of preschool age (1 month to 5 years old) who lived in three high-risk districts for polio, and assigned them equally to three groups.
In the first group, children received only OPV (Fig. 1). The second group was offered an integrated health package, consisting of educational materials for the community, OPV and other immunizations and nutritional supplements, provided at fixed and mobile health camps. The third group was offered the same package as the second, but with the addition of IPV. The researchers found that integrated health packages led to better community acceptance of polio vaccination and, consequently, coverage with OPV increased by 6.6% in the second group and 8.5% in the third group compared to the first.
Habib and colleagues' study was conducted between June 2013 and May 2014, and its integrated approach has already been deployed in other areas of conflict in Pakistan. In those regions, the polio situation overall has improved in the past three years, owing to better epidemiological surveillance and improved security for the teams of health-care workers who administer vaccinations. As a result, only three cases of polio have been reported in Pakistan so far this year (as of 12 July). The challenge, however, remains in targeting cross-border polio transmission between Pakistan and Afghanistan, the last two places on Earth where the transmission of wild poliovirus is still documented. (Nigeria has not reported any such cases since August 2016.)
Community involvement will be essential if vaccination is to be accepted in these regions. Otherwise, many poor populations will not see the benefit of receiving a dose of vaccine in the absence of other much-needed health interventions. As Habib et al. propose, immunization programmes must become the backbone for delivering other cost-effective interventions to the community if acceptance rates are to improve.
In addition to implementation challenges, technical barriers to the eradication of polio remain. The OPV used in the main push to eradicate polio consisted of three strains of attenuated virus, but a well-coordinated transition has succeeded in making use of an OPV that contains only two6. This should eliminate the occurrence of polio outbreaks resulting from mutation of the removed strain, which was the most common cause of cVDPV.
Another technical issue is that the enormous demand for IPV has surpassed the current production capacity, leading to a temporary shortage. A few countries are overcoming this by using a fractional dose of IPV administered into the skin, with good results. Transmission by infected individuals must also be tackled — for this, effective antivirals will need to be identified, and several are being tested. Finally, the production of IPV involves making an inactivated strain from wild poliovirus, which has the potential to escape from the laboratory. A transition to generating IPV with the same attenuated strains as those used for OPV will reduce the risk of such an occurrence.
Every year, the Global Polio Eradication Initiative helps to vaccinate 450 million children against polio. According to the World Health Organization, these efforts are estimated to have saved the lives of 1.5 million children, and to have prevented paralysis in 16 million people. The eradication of polio is arguably the world's most ambitious global-health target, and is one that can be achieved only after the challenges and technical hurdles that still hamper its implementation have been tackled. We are very close, and the dividends will be enormous. Let's finish the job, once and for all.
Jaime Sepulveda is executive director of Global Health Sciences at UCSF and the Haile T. Debas Distinguished Professor of Global Health Science. This article appeared originally in Nature.
- Habib, M. A. et al. Lancet Glob. Health 5, e593–e603 (2017).
- Patel, M. & Orenstein, W. N. Engl. J. Med. 374, 501–503 (2016).
- Sepúlveda, J. et al. Lancet 368, 2017–2027 (2006).
- Daniel, T. M. & Robbins, F. C. Polio (Univ. Rochester Press, 1997.
- Hsu, C. H. et al. MMWR Morb. Mortal. Wkly Rep. 65, 1295–1299 (2016).
- Global Polio Eradication Initiative. Polio Eradication and Endgame Strategic Plan 2013–2018 (World Health Organization, 2013).