Six days ago, New York declared a state of emergency over polio after samples were found in wastewater in New York City and four adjacent counties. Polio is a highly infectious disease caused by a virus that can enter the nervous system. By doing so, it causes paralysis very quickly. The polio virus spreads from person to person through the faecal-oral route and less frequently via contaminated water or food. Polio mainly affects children under five years of age and the number of wild cases has decreased by over 99% since 1988. There is no cure for the disease and nowadays it is mainly tackled through vaccination.
There are two vaccines available: oral polio vaccine and inactivated polio vaccine. These were introduced in the mid-20th century when over half a million of people were still getting infected every year – one of them being my father, whose leg was affected but luckily not severely. It is visible but he has been able to carry on with his normal activities.
Inactivated and oral polio vaccines
The first vaccine produced was the inactivated polio vaccine (IPV). It is interesting to note that when its creator, Jonas Edward Salk, was interviewed in 1955 and asked who should own the patent for it, he replied: “Well, the people, I would say. There is no patent. Could you patent the sun?” He didn’t receive any profit from it. Can you imagine this in the world in which we live today? Although the vaccine was quite efficient in protecting children, it didn’t stop virus transmission.
The second type of vaccine, developed by Albert Sabin and Mikhail Chumakov, is the oral polio vaccine (OPV). As the name suggests, it is given orally. It is a live-attenuated vaccine that contains a weakened form of the virus. Its ease in administering made it the ideal candidate to be used worldwide. Furthermore, it had the advantage of interrupting the chain of transmission and stopping outbreaks. However, there is a drawback regarding this vaccine – continued use of it puts the elimination of the disease at risk.
In areas where the vaccine uptake is low, the weakened vaccine virus present in OPV can begin to circulate in under-vaccinated communities. If this circulation is allowed for a long enough time, the virus can mutate into a stronger virus and end up causing what is known as circulating vaccine-derived polioviruses (cVDPVs).
Keeping our eyes open for cVDPVs
While wild poliovirus circulates in Afghanistan and Pakistan, cVDPVs appear elsewhere from time to time, mainly in Africa and Asia.
The United States and United Kingdom use IPV instead of OPV and there is a high polio vaccination rate in these countries, meaning that children should be protected from the effects of the virus. However, these viruses have the ability to find unvaccinated individuals. As an example, in the 1990s there was an outbreak in the Netherlands, where the overall vaccination coverage was more than 90%, resulting in two deaths and 59 cases of paralysis.
More recently, cVDPVs have been found in New York, Jerusalem, and the United Kingdom. Consequently, scientists have become concerned. In the UK, wastewater surveillance is common in London and in Glasgow. The virus has been detected in London’s sewage since February and no associated cases of paralysis have been found. Also, it seems the virus is contained in the north and east of London.
Avoiding a resurgence
What should the next steps be to avoid a resurgence in the number of cases due to the appearance of different cVDPVs? We know very well that travelling makes the world smaller and is a significant vector in virus spread. There have been many outbreaks in developing countries in these last years, with a peak in 2020, when more than 1,100 cases of vaccine-derived polio were reported.
The UK, USA, and Israel have made a huge effort to vaccinate all one to nine-year-olds. However, those countries with a high volume of travel and contact with polio-affected countries must strengthen surveillance to detect the import of new viruses and facilitate a rapid response.
Uniformly high routine immunisation coverage is paramount. Will the UK be willing to do this after the experience we have had with Covid? Will it be able to attend to this demand, despite the crisis we are now living through within the health service? During the Covid-19 pandemic, UKHSA reported that, in west London, only 35% of teenagers received the booster for polio in 2020–21.
Questions still to be answered
What happens if the virus detected in London’s sewage is not contained in the areas it was discovered? What must be done to prevent new harmful mutations of the virus and consequently more transmissions?
A new vaccine against cVDPVs – nOPV2 – received emergency approval by the WHO in late 2020. This vaccine is also derived from the live virus, but through genetic engineering. The regions of the genetic code that were mutating and making the virus regain virulence have been swapped for others at strategic points. Other alterations were also carried out so the virus doesn’t recombine with others in the gut, and therefore will take longer to evolve.
Questions still have to be answered though. Will this vaccine be completely safe from generating other cVDPVs? How widespread will the coverage with this new vaccine be? And what efforts will be taken to administer it separately from the other vaccines?
