Determinants of monovalent oral polio vaccine mutagenesis in vaccinated elderly people

Live oral poliovirus vaccine (OPV) strains can mutate and recombine during replication in the host. Trivalent OPV has long been used to restrain wild-type poliovirus in developing countries. However, recently WHO advocates using monovalent OPV (mOPV) to finally eradicate poliovirus world-wide. We analysed polioviruses recovered from the faeces of 101 elderly patients (divided into three groups by immune status) challenged with mOPV-1 or mOPV-3. A high number of nucleotide mutations was found in the viral capsid-protein-encoding regions. Some of these mutations caused amino acid changes in or near regions with neutralizing epitopes, especially in mOPV-1-derived strains. The quantities of mutations in recovered poliovirus strains correlated with prevaccination immune status (seronegatives have more mutations) and excretion duration. Duration of excretion appears to be the dominant factor for the accumulation of mutations in mOPV-derived strains in vaccinated elderly people.