Hazard Characterization of Modified Vaccinia Virus Ankara Vector: What Are the Knowledge Gaps?

Okeke, Malachy I.; Okoli, Arinze S.; Diaz, Diana; Offor, Collins; Oludotun, Taiwo G.; Tryland, Morten; Bøhn, Thomas; Moens, Ugo

In order to reach the scale required for current GMP of MVA-vectored vaccines intended for clinical application, the vaccine needs to be amplified multiple times. Such high titer amplification creates a selective pressure that may result in instability of the recombinant MVA. Instability may occur within the transgene expression cassette (transgene instability) or outside the expression cassette (genome instability). Provision of genome sequences of MSV for up to five passages is necessary in order to evaluate the genome stability of recombinant vaccines based on MVA. However, provision of whole genome sequences is not mandatory under Directive 2001/18/EC. Thus, the genome stability of all MVA-vectored vaccines intended for clinical application remains unknown and potential mutations outside the expression cassette and the phenotypic implications of such mutations go unreported. Transgene stability of recombinant MVA-vectored vaccine is routinely examined during hazard characterization. In 2004, our group reported that the influenza virus hemagglutinin (HA) transgene in some CPXV-MVA recombinants is unstable resulting in the loss of the transgene following serial passages [95] and southern blot analysis suggested that the HA transgene in the first recombinant MVA-vectored vaccine is unstable [95] . Other groups have also reported transgene instability in some MVA-vectored vaccines [102] [103] [104] . Transgene stability is essential for robust ERA since the transgene is the logical tag for tracking the spread of the vaccine virus from target recipients to non-target species. Consequently, the loss of the transgene may preclude the monitoring of non-target effects from released or escaped recombinant MVA vaccines.