Vaccine-derived poliovirus

Polio is an incurable disease. However, the effective oral polio vaccine, also called OPV or polio drops, has been used globally to effectively prevent the spread of the poliovirus. The vaccine contains weakened strains of the poliovirus, which, upon being administered, helps the body develop immunity against polio. 

Despite all this, there remains a cause for worry. Polio drops are given to children under the age of five years. The weakened version of this virus then replicates itself in the intestines and enters the bloodstream, following which an immune response is triggered.

As in the case of wild poliovirus, the vaccinated child excretes this weakened virus for a period of six to eight weeks. The vaccine virus can, hence, continue to spread from one child to another via the faecal-oral route (for example, children might play in the dirt or touch contaminated surfaces and eat without washing their hands or suck on their thumb, etc.). This can turn into a counterproductive situation especially when a large number of children are not vaccinated and immunity levels are low.

When the virus travels from one unprotected child to another, it can undergo genetic mutations over time. Eventually, the attenuated (weakened) virus can mutate to a form that can cause paralysis. This genetically mutated virus that forms from the strains present in the oral vaccine is called vaccine-derived poliovirus (VDPV).

Three kinds of vaccine-derived polioviruses (VDPVs) have been identified so far. These are immunodeficiency-related VDPVs, circulating VDPVs and ambiguous VDPVs.

• Immunodeficiency-related vaccine-derived polioviruses: When a small population of people with rare immune deficiency disorders contract VDPV, a prolonged replication takes place. In ideal circumstances, this infection would clear out within six to eight weeks. However, due to suppressed immunity (weak immunity), these people are not able to clear out the vaccine-derived poliovirus infection.
  This is an extremely rare form of VDPVs and just over 100 cases have been observed since the 1960s.
• Circulating vaccine-derived polioviruses: In an under-immunized population, especially that involving susceptible children, the excreted mutation is able to circulate for over 12-18 months. In the past 10 years, around 800 cases of circulation VDPVs or cVDPVs have surfaced.
• Ambiguous vaccine-derived polioviruses: This is the third type of VDPVs. Very little is known about them, hence the name ambiguous vaccine-derived polioviruses or aVDPD. aVDPVs are either isolated from people with no known immunodeficiency disorders or they come from sewage.

Up until May 2020, around 150 cases of VDPVs have been reported in India. Every two out three of these cases were reported between 2010 and 2019. This accounts for a staggering 66%. Out of these cases, 59% reportedly occurred in children under the age of two years. The cases were detected during a survey for Acute Flaccid Paralysis. According to the report, this increase was because of increased identification and the formulation of better techniques for detecting poliovirus.

India has achieved its polio-free status after a lot of hard work, consistent effort and commitment of resources—it was obviously tremendously challenging owing to the vast population and the climatic conditions of the country. The World Health Organization, Global Polio Eradication Initiative, UNICEF and the Indian government all worked together for this. A lot of this work could be undone or even go to waste if we become lax now and don’t get our children vaccinated against poliovirus.

The management of all polioviruses, irrespective of where the strain comes from, is the same. That is, vaccination of every child multiple times to prevent the transmission of polio. When the population is fully and completely immunized, it will be well protected against both wild and vaccine strains of poliovirus.

Once the complete eradication of wild poliovirus is achieved globally—and after the death of the last person with polio infection—VDPVs will be the only strain with the potential for re-emergence of polio. At that stage—when there are no other polioviruses left in the world—scientists might suggest phasing out oral polio vaccine.