Aside from recent outbreaks of polio in war-torn regions of the world, the deadly virus is close to being eradicated , thanks to vaccines.
Authors
- Lee Sherry
Postdoctoral Research Associate, School of Infection and Immunity, University of Glasgow
- Nicola Stonehouse
Professor in Molecular Virology, School of Molecular and Cellular Biology, University of Leeds
All vaccines work by training our immune systems to recognise a harmless piece of a virus or bacteria so that when the real thing is encountered later, the immune system is prepared to defeat it.
There are two types of polio vaccine in use. One is the inactivated poliovirus vaccine (IPV), and the other the live-attenuated oral poliovirus vaccine (OPV).
The IPV is made by "killing" large quantities of poliovirus with a chemical called formalin, making it unable to replicate. The immune system is then "trained" to recognise the poliovirus - which is thankfully rendered safe by formalin.
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The OPV vaccine contains a weakened (or "attenuated") version of the virus. These changes in the virus's genetic code stop it from causing disease. However, as the OPV vaccine is still capable of replicating, it can revert to a form that can cause disease, with the potential to cause paralysis in unvaccinated people.
Because of these risks, scientists are now looking for safer ways to create vaccines - methods that don't require growing large amounts of the live virus in high-security labs, as is done for IPV.
Our research team has taken an important step towards producing a safer and more affordable polio vaccine. This new vaccine candidate uses virus-like particles (VLPs). These particles mimic the outer protein shell of poliovirus, but are empty inside. This means there is no risk of infection, but the VLP is still recognised by the immune system, which then protects against the disease.
This vaccine candidate uses technology that's already being used in hepatitis B and human papillomavirus (HPV) vaccines. Thanks to VLPs, since 2008, there have been no cervical cancer cases in women in Scotland who were fully vaccinated against HPV. Over the past ten years, our research group has worked to apply this successful technology in the fight to eradicate polio.
Vaccine success
Throughout the 19th and 20th centuries, polio was a major global childhood health concern. However, the development of IPV (licensed in 1955) and of OPV (licensed in 1963), almost eliminated polio-derived paralysis. Due to the success of the Global Polio Eradication Initiative , introduced in 1988, most cases of paralytic polio are now caused by the vaccine.
Despite the success of these vaccines, they both have safety concerns that could threaten to compromise eradication of the disease.
IPV, for instance, is expensive to make because it needs stringent safety measures to prevent the accidental release of live poliovirus and so is mostly used in wealthy countries. OPV is five times cheaper than IPV, and due to its lower cost and ease of use, it is used almost exclusively in developing countries.
OPV has been instrumental in the near eradication of "wild polioviruses" (the naturally occurring form) around the world. But in areas where vaccination rates are low and enough people are susceptible to infection, the weakened virus (OPV) can replicate .
Unfortunately, each round of replication increases the potential for the virus to revert to a form of polio that causes illness and paralysis. This is already evident in new vaccine-derived outbreaks across several countries in Africa, Asia and the Middle East, which now accounts for most paralytic polio cases worldwide . So, once all remaining strains of wild poliovirus have been successfully eradicated, OPV use will have to stop.
Safer vaccine
The next generation of polio vaccinations is likely to be produced in yeast or insect cells. Our research shows that VLPs produced in both yeast and insect cells can perform equally or better than the current IPV.
These non-infectious VLPs are also easier to produce than IPVs. They would not need to be handled under such stringent laboratory conditions as IPVs, and they are more temperature stable, thanks to genetic alteration of the outer shell. The new vaccines, then, will be less expensive to produce than IPVs, helping to improve fair and equal access to vaccination - ensuring that once polio is eradicated, it will stay eradicated.
As we move closer to wiping out polio worldwide, these next-generation vaccines could be the final tool we need - safe, affordable and accessible to all.
Lee Sherry worked as a post-doc on a WHO-funded research grant for the production of poliovirus virus-like particles
Nicola Stonehouse is a member of the WHO VLP vaccine Consortium and receives funding from The World Health Organisation - Generation of virus-free polio vaccine.
