Fast track — ArticlesProtective efficacy of a monovalent oral type 1 poliovirus vaccine: a case-control study
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 transmission was the result of a failure to immunise a sufficiently high proportion of children against poliomyelitis.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 international 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 [CCID50] vs 200 000 CCID50 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, 11, 12 In Egypt, no indigenous 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.
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Patients and procedures
Since the introduction of mOPV1 use in India in 2005, vaccination efforts have focused on the northern states of Uttar Pradesh—where over 80% of all type 1 cases of poliomyelitis in India in 2006 occurred—and Bihar. Frequent rounds of vaccination with mOPV1 have been interspersed with use of trivalent vaccine to maintain immunity to type 3 poliovirus. In the few districts with continued reporting of type 3 poliomyelitis, monovalent vaccine against type 3 (mOPV3) has also been used in up to two
Results
122 173 cases of acute flaccid paralysis were identified. Of these, 2580 did not have two adequate stool samples and had residual paralysis compatible with poliomyelitis and were thus excluded from the analysis; a further 5773 cases did not report the number of vaccine doses received and were also excluded. 4966 cases of type 1 poliomyelitis had complete dose information for the entire study period; of these, 2076 were matched with suitable controls (table 1). The age distribution of matched
Discussion
Our results show that, in the state of Uttar Pradesh, the monovalent vaccine is about three times more likely to result in a protective immune response against type 1 paralytic poliomyelitis than is the trivalent vaccine, irrespective of the assumption about routine immunisation. This increased efficacy is probably caused by the absence of interference between the three Sabin vaccine strains.20 Even balanced formulations of trivalent poliovirus vaccines tend to result in preferential infection
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Polioviruses
2018, Principles and Practice of Pediatric Infectious Diseases