Higher-potency mOPV1 in India

Following the development and licensure of monovalent oral polio vaccine type 1 (mOPV1) in December 2004 in India, massive quantities (>2 billion doses) of this vaccine have been used in northern India (Uttar Pradesh and Bihar) for polio eradication. The public health impact of the use of this vaccine in the targeted areas has been very rewarding, with no indigenous cases of poliomyelitis caused by poliovirus type 1 detected in western Uttar Pradesh for a period >12 months, thus demonstrating the biological feasibility of eradication in this most difficult-to-eradicate area in the world. In short, endemic transmission of type 1 poliovirus was successfully interrupted from western Uttar Pradesh. However, in mid-2008, poliovirus type 1 was re-imported into some areas of western Uttar Pradesh from neighbouring Bihar. As this (ultimately successful) mOPV1 strategy was being carried out in western Uttar Pradesh, further clinical data that would demonstrate empirically the increased efficacy of this specific mOPV1 vaccine was gathered, as two clinical trials were initiated in 2005 and 2006.

In early July 2008, the trial investigators met in Vellore, India to discuss the preliminary results of the two clinical trials conducted in Indore (Madhya Pradesh) and Hyderabad (Andhra Pradesh) during the past three years. The trials evaluated regular-potency mOPV1 and higher-potency mOPV1, compared to trivalent OPV (tOPV), all formulated by Panacea Biotec Ltd. In addition, the second trial included an international comparison mOPV1 produced by Sanofi Pasteur, Lyon, France, which had been used in an initial clinical trial in Egypt in 2005 (N Engl J Med 2008, 359;16; 1655-1665). The results of the Indore and Hyderabad trials were almost identical and demonstrated that:

1. a birth dose of polio vaccines (mOPV1 or tOPV) given shortly after birth results in low seroconversion rates (between 10-18% against poliovirus type 1 in Indian newborns)
2. a dose of mOPV1 given at 30 days of age, in contrast, results in unexpectedly high seroconversion rates (>85% in the mOPV1 arms compared with almost 60% of trivalent OPV) against poliovirus type 1
3. the higher-potency mOPV1 did not result in appreciably higher immunity gains than the regular-potency mOPV1.

The causes for the lower immunogenicity in Indian newborns are not known, but do not appear to be related to vaccine potency or biological factors, such as levels of maternally-derived antibodies or timing of feeding breast milk (colostrum). The findings of these trials confirm the superiority of OPV1 over trivalent OPV in inducing immunity against poliovirus type 1 and support the continued widespread use of this vaccine in children <5 years of age to facilitate the elimination of the last chains of poliovirus type 1 transmission in India.