Immunogenicity of oral polio vaccine in different doses in Mozambique

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The attenuated poliovirus strains contained in oral polio vaccine (OPV) can re-acquire the neurovirulence and transmissibility of wild poliovirus in populations with low immunity. Because of the significant burden in paralytic cases caused by outbreaks of circulating vaccine-derived poliovirus (cVDPV), it was decided to switch from trivalent to bivalent OPV containing only type 1 and 3 polioviruses, and incorporate a dose of inactivated poliovirus vaccine (IPV) in routine immunization schedules. Type 2 monovalent OPV (mOPV2) was reserved for use in response to potential VDPV2 outbreaks after the tOPV-bOPV switch in April 2016. Unfortunately, the number and size of VDPV2 outbreaks observed after the switch has exceeded expectations and the mOPV2 available in the stockpile may be insufficient. A potential solution to stretch the mOPV2 stockpile would be giving 1 drop instead of the conventional 2 drops. The recommended content of Sabin poliovirus in a 2-drop dose of mOPV2 is at least 105 TCID50, but most batches include 2-4 times the minimum recommended amount. Therefore, half the dose of mOPV2 (i.e. 1 drop) could still induce appropriate immunogenicity. However, policy makers need clinical trials among children naïve to type 2 OPV before recommending a reduction in mOPV2 dose and trials can only be done in countries where mOPV2 can be legally used, which is those experiencing a VDPV2 outbreak, such as Mozambique. We will conduct a clinical trial in Mocuba district, Zambezia, Mozambique that will compare the immunogenicity against type 2 poliovirus of one drop versus two drops of mOPV2. The results of this study will guide the Global Polio Eradication to develop strategies that prevent a devastating shortage in mOPV2.