Current endgame strategies for polio set out by the World Health Organization (WHO) involve a mass switchover from the currently used oral polio vaccine (OPV) to the inactivated polio vaccine (IPV).
The OPV is still widely used across the globe, including in India. It has been one of the most essential tools in the global fight against polio.
However, the switchover to IPV may be the final nail in the coffin for polio. However, there are currently a number of factors limiting this mass switchover These include the greater expense of the IPV, lower capacity for production and alterations in supply chains.
First attempts at eradication and introduction of the OPV
India is a prime example of a successful polio eradication campaign. India’s geographical size, the difficulty of reaching rural locations and populations, poor sanitation coverage and high population density meant it was a challenge. Nonetheless, India eradicated wild-type polio. Its last reported case was in 2011 and the WHO declared India polio-free in 2014.
This came despite India having the highest number of polio cases in the world as late as 2009. The dramatic turnaround was achieved through the tireless efforts of 2.3 million polio volunteers and 150,000 supervisors across the country. Such a large-scale effort is an achievement in and of itself.
Much of their success can be attributed to the ease of use of the OPV. As a simple oral tablet, dosages of the vaccine could be administered by practically anyone, with very little training or equipment required. This allowed teams across the country to head to rural areas and administer the vaccination.
The earliest attempt to isolate poliovirus was by Dr C G Pandit, the first director of the Indian Council of Medical Research (ICMR). The Expanded Programme on Immunization – a precursor to the Universal Immunization Program (UIP) – was started in 1978 which provided the OPV to children as part of the programme.
In May 2006, the monovalent oral polio vaccine type 1 (mOPV1), was introduced, as a targeted intervention at birth in Western Uttar Pradesh. mOPV1 was recommended because of its greater efficacy against WPV1 when compared with trivalent OPV due to the body focusing antibody production against only one strain.
While the use of mOPV1 in most polio rounds had helped reduce type 1 poliovirus transmission to record low levels, India experienced an outbreak of type 3 poliovirus in 2009. There were 659 cases of predominantly type 3 poliovirus infections and 15 cases of vaccine-derived poliovirus infection. About 80% of them occurred in Uttar Pradesh and 17% in Bihar.
The last polio case in India was reported in 2011 in a two-year-old girl, Rukhsar Khatun in Howrah, West Bengal. Notably, her two siblings received polio vaccinations. Rukhsar, however, was often sick with diarrhoea. Despite encouragement from local health workers, her parents had thought it was safer for her to avoid the vaccine.
In response to the lone case of polio in 2011 in Howrah, a large-scale mop-up immunization drive was launched within seven days of notification of the case. This targeted action was successful and India was finally removed from the list of polio endemic countries after completing a year without reporting any case of polio in January 2012.
Why switch to IPV?
If the OPV has been such a successful tool, why is this changeover necessary?
As mentioned in the case of the 2009 outbreak of type 3 poliovirus, fifteen cases were vaccine-derived. This is one of the critical downsides to the OPV. The strain of polio used in the vaccine is still living, though in a severely weakened state.
The OPV uses this weakened strain of polio — which proceeds to replicate in a recipient’s intestines — to allow antibodies in the host to be produced against the virus. This typically causes no symptoms. However, in very rare cases the OPV can cause vaccine-associated paralytic poliomyelitis – paralysis which is identical to that caused by the original polio virus.
While cases are exceptionally rare, vaccine-derived polio presents a persistent threat to polio eradication efforts. Immunocompromised individuals are primarily at risk. In 2013, a death related to vaccine-derived polio was found to have occurred in a severely immunocompromised child.
During the time in which the vaccine-derived polio is replicating in the host’s intestines, the vaccine-virus is also excreted. In areas of inadequate sanitation, particularly where open defecation is practised, this excreted vaccine-virus can spread in the immediate community (conferring protection to others in the community through ‘passive’ immunization), before eventually dying out.
If a population has a low level of immunisation, this vaccine-virus can circulate for an extended period of time. With every replication of the virus, there is a risk that mutations to its genome can occur. The longer the virus remains in circulation the greater the risk that it may mutate into a strain that is again capable of causing paralysis. Vaccinated individuals are still immune to this strain, making the issue only a concern in communities with low levels of immunisation coverage.
Since 2000, more than ten billion doses of OPV have been administered to nearly three billion children worldwide. During this time, 24 cVDPV outbreaks occurred in 21 countries, resulting in fewer than 760 VDPV cases.
Crucially, these vaccine-derived cases do not occur when using the IPV. The IPV contains all three serotypes of polio. These strains have been killed off using formalin, preventing any future mutation and so do not present any possibility of future infections.
Global shortages of IPV
Removing the threat of vaccine-derived polio is essential to the end-game strategy. The implementation of the IPV is the next major step to this. If this is the case then why do so many developing nations still rely on the OPV?
It has already been demonstrated that the global community is capable of mass-scale vaccine alterations. Over a two-month period across April and May 2016, 155 nations using a trivalent oral polio vaccine (OPV) organised a mass switch to a bivalent OPV. Though little attention was given to this by the media, the achievement was considerable.
It was found that around 90 percent of cVDPV cases were caused by the type 2 poliovirus strain within the vaccine, making this switchover a logical progression to eliminating the threat of vaccine-derived polio. As of 2015, wild type 2 poliovirus was declared eradicated. The last known case in India occurred in 1999.
Global implementation of the IPV may however be delayed simply by manufacturing capacities. “We may not have enough IPV for the whole world to use,” said Michel Zaffran, the World Health Organization’s director of polio eradication in an interview with STATnews last year.
Long-term sustainable planning is a necessity in this regard, as global shortages could feasibly occur in a number of years time when the global community has shifted to exclusively using IPV. This could cause gaps in immunisation within countries with low stockpiles, opening opportunities for resurgences in disease cases.
Currently, the IPV is predominantly used in developed nations where polio has long since been eradicated, typically as part of routine vaccinations in children. Developing nations are still almost entirely reliant on OPVs.
To roll out IPVs to every nation using polio vaccines would require considerable investment. The IPV is more expensive than the cheaply produced OPV. For developing nations this could present issues if the funding is not in place to support continued polio vaccination programmes.
Issues may also arise from the more technical aspects of using an injectable vaccine. The simplicity of the OPV being an oral vaccine is what allowed untrained individuals to deliver and administer vaccines to many out of reach areas. More highly trained individuals may be necessary to administer an injection, presenting possible issues in developing nations with a lack of local expertise.
The potential for vaccine-derived polio in India even after eradication
Despite complications and costs associated with a switch to IPV, it is the only assured way of preventing further outbreaks of vaccine-derived polio.
Naturally occurring polio cases worldwide have dropped to just a handful of infected individuals. These cases are isolated to just two countries, Pakistan and Afghanistan. As Polio has all but faded into obscurity its media profile has diminished.
While OPV is still in use this presents a dangerous scenario. It is entirely possible that as polio fades away from public memory, fewer and fewer individuals will ensure that their children are vaccinated. In India, where much of the country still practises open defecation, this could allow for any individual who does receive the OPV to circulate the vaccine-virus among the community. As previously mentioned, this allows for mutations that could make the virus harmful again.
Once wild cases of polio are officially eradicated, this possibility becomes ever more plausible. Many may think that, as polio has been eradicated, it is no longer a threat. Therefore why are vaccinations still required?
However, polio could yet show a resurgence. If immunisations rates fall this is a far greater threat. It may be the case that isolated infections are still present in remote areas that have not been picked up by surveillance systems. As cases are still present in India’s neighbouring Pakistan, India would be at particular risk from this.
Other instances such as an accident at a vaccine plant — previously seen in Belgium in 2014 that pumped polioviruses into a river and the North Sea — could release the viruses back into the world.
While the polio campaign may be in its endgame, many factors must still be taken into account before the disease can truly be considered gone for good. The switchover of vaccines to eliminate threats of vaccine-derived disease is one. Maintaining surveillance programmes and vaccine production capacity long after the eradication announcement is another. Huge strides have been made towards eradication, but the campaign is not yet over.