Poliovirus causes a debilitating and sometimes fatal paralytic disease, poliomyelitis, especially in infants and young children. There is no treatment, but safe and effective vaccines have been available since the 1950s. At one time polio was prevalent in many parts of the world but with the widespread use of polio vaccines, polio is very close to being completely eradicated from most of the countries. Soon the world will be polio free, and it will be critical to safeguard this eradication. Scientists are concerned that current vaccines could lead to the reemergence of polio virus in a polio-free world, and there is a need to develop safer strains of polio that can be used in future vaccines.
In a recently published study in PLoS Pathogens, Sarah Knowlson and colleague show that they have designed extremely genetically stable, and hyper-attenuated polio strains, which cannot revert to a dangerous wild-type form. The authors believe that these strains will be ideal candidates to develop vaccines for polio. The study shows that these new strains not only produce acceptable virus titers in culture but also have the same antigenic and immunogenic properties as the parent strains.
There are two major types of polio vaccine that are currently used: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). Inactivated vaccines (also called Salk IPV) use inactive or killed polioviruses and are given by injection. Oral polio vaccines (also called Sabin vaccines) are live vaccines that are attenuated /weakened my multiple passages in culture are as the name denotes are given orally. Both vaccines are used currently with OPV being more used in areas where polio is endemic. However, both vaccines are not suitable for long-term use; Sabin or OPV strains can revert to wild type strains, and production of IPV requires growth of large amounts of wild-type viruses thus posing a security risk.
Sarah Knowlson and colleagues at the Department of Virology of National Institute for Biological Standards and Control used the attenuated Sabin type3 vaccine as a starting strain and generated four new successively weaker strains by introducing some changes in viral RNA. To examine if these strains are fit for vaccine production the authors compared them to the original Sabin strain and the IPV strain. They compared growth properties, genetic stability and neuro-virulence of these new strains through different experiments. Their results show that these new viruses produce acceptable viral titers in culture and are genetically stable. They also noted that these strains were safe to use, as they did not cause paralysis in the mice. WHO has recently recommended that once the global eradication of polio is confirmed, the use of all polio vaccine will be ceased to minimize reintroduction of polio in a polio-free world. The authors believe that the new strains that they have developed could be used as a potential vaccine in future after further testing to confirm their genetic stability and low neuro-virulence.