Articles www.thelancet.com Published online April 12, 2007 DOI:10.1016/S0140-6736(07)60531-5 1 Protective efficacy of a monovalent oral type 1 poliovirus vaccine: a case-control study Nicholas C Grassly, Jay Wenger, Sunita Durrani, Sunil Bahl, Jagadish M Deshpande, Roland W Sutter, David L Heymann, R Bruce Aylward Summary Background A high-potency monovalent oral type 1 poliovirus vaccine (mOPV1) was developed in 2005 to tackle persistent poliovirus transmission in the last remaining infected countries. Our aim was to assess the efficacy of this vaccine in India. Methods We estimated the efficacy of mOPV1 used in supplementary immunisation activities from 2076 matched case-control pairs of confirmed cases of poliomyelitis caused by type 1 wild poliovirus and cases of non-polio acute flaccid paralysis in India. The effect of the introduction of mOPV1 on population immunity was calculated on the basis of estimates of vaccination coverage from data for non-polio acute flaccid paralysis. Findings In areas of persistent poliovirus transmission in Uttar Pradesh, the protective efficacy of mOPV1 was estimated to be 30% (95% CI 19–41) per dose against type 1 paralytic disease, compared with 11% (7–14) for the trivalent oral vaccine. 76–82% of children aged 0–23 months were estimated to be protected by vaccination against type 1 poliovirus at the end of 2006, compared with 59% at the end of 2004, before the introduction of mOPV1. Interpretation Under conditions where the efficacy of live-attenuated oral poliovirus vaccines is compromised by a high prevalence of diarrhoea and other infections, a dose of high-potency mOPV1 is almost three times more effective against type 1 poliomyelitis disease than is trivalent vaccine. Achieving high coverage with this new vaccine in areas of persistent poliovirus transmission should substantially improve the probability of rapidly eliminating transmission of the disease. Introduction By early 2004, the transmission of indigenous wild poliovirus had been interrupted in all but six countries of the world as a result of a concerted international eradication effort. 1 In four of these countries—Nigeria, Niger, Pakistan, and Afghanistan—sustained trans- mission was the result of a failure to immunise a sufficiently high proportion of children against polio- myelitis. 2 However, In India and Egypt, poliovirus transmission persisted despite immunisation coverage with four doses of the trivalent oral poliovirus vaccine of more than 90% among children aged less than 5 years. 3,4 In recognition of the grave threat that persistent transmission in India and Egypt posed to the Global Polio Eradication Initiative, the programme’s inter- national oversight body urgently reviewed a range of options in October, 2004, to enhance the effectiveness of vaccination in these areas. By that time, transmission of wild type 2 poliovirus had been interrupted worldwide and type 3 poliovirus had been eliminated in Egypt and all but one state of India. Consequently, the Advisory Committee on Polio Eradication recommended the rapid development, licensing, and introduction of a new monovalent oral type 1 poliovirus vaccine (mOPV1). 1 This new vaccine possesses five times the potency of licensed monovalent vaccines used in the early 1960s (1×10⁶ median cell culture infective doses [CCID 50 ] vs 200 000 CCID 50 per dose). 5 Through an extraordinary public-private development effort this new mOPV1 was licensed by April, 2005, in India and Egypt and used in mass polio immunisation campaigns in India (April, 2005) and Egypt (June, 2005). 6,7 The efficacy of mOPV1 has major implications for international public health. The Global Polio Eradication Initiative has invested US$5 billion in eradication over a 20-year period and a key role is now proposed for monovalent vaccines in the strategic approach to interrupting the transmission of remaining indigenous wild poliovirus and managing the risks of re-emergent transmission of poliovirus after global certification of eradication. 8,9 Especially important to the programme is the effectiveness of the monovalent vaccine under field conditions of poor sanitation and high population density, where a high prevalence of diarrhoeal disease and other infections have been shown to interfere with the efficacy of trivalent oral poliovirus vaccine as well as to favour the transmission of wild poliovirus. 10–12 In Egypt, no indi- genous strain of wild poliovirus has been detected since the introduction of mOPV1. 6 In India, however, a polio outbreak in 2006 allowed us to study the efficacy of this new vaccine under field conditions. Our aim was to determine the protective efficacy of mOPV1 in India and explore the consequent implications of mOPV1 for global polio eradication and post-eradication risk management. Methods Patients and procedures Since the introduction of mOPV1 use in India in 2005, vaccination efforts have focused on the northern states of Published Online April 12, 2007 DOI:10.1016/S0140- 6736(07)60531-5 See Online/Comment DOI:10.1016/S0140- 6736(07)60533-9 Department of Infectious Disease Epidemiology, Imperial College London, London, UK (N C Grassly DPhil); National Polio Surveillance Project, WHO, New Delhi, India (J Wenger MD, S Durrani BSc, S Bahl MD); Enterovirus Research Centre, Parel, Mumbai, India (J M Deshpande PhD); and Global Polio Eradication Initiative, WHO, Geneva, Switzerland (R W Sutter MD, D L Heymann MD, R B Aylward MD) Correspondence to: Dr Nicholas C Grassly, Department of Infectious Disease Epidemiology, Imperial College London, Norfolk Place, London W2 1PG, UK n.grassly@imperial.ac.uk