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Why is the oral polio vaccine still crucial to eradicate polio?

GS Paper 2

 Syllabus: Issues related to health

 

Source: TH

Context: It seems that the last mile to realising the dream of a polio-free world is going to be challenging.

 

Why?

  • The US, UK and Israel recently reported polio cases in unvaccinated people after having been polio-free for more than a decade.
  • Their silent outbreaks in countries with excellent sanitation and hygiene infrastructure have raised eyebrows, adding a new dimension to the Global Polio Eradication Initiative (GPEI).

Polioviruses: These are enteroviruses that are transmitted primarily by the faecal-oral route. There are different types of polio vaccines available.

 

The oral polio vaccine (OPV): It is a live attenuated vaccine, that contains weakened polioviruses (all three types – 1, 2, and 3) to induce an immune response in the human body without causing disease.

 

The advantages of the OPV:

  • Inexpensive and easy to administer to a large population and can help prevent transmission.
  • It triggers a robust immune response in the gut lining (called the mucosal immune response) in addition to the significant immune response in the blood (the systemic immune response).
  • It is excellent at preventing disease as well as person-to-person transmission of the virus.
  • It is also easier to manufacture as the “seed virus” required for the vaccine is an attenuated (weakened form) virus.

 

Drawbacks of the OPV:

  • Its ‘uptake’ by the immune system after being administered is not as good as is desirable, especially in developing countries like India.
  • The OPV can reverse the virus’s neurovirulence in rare cases, leading to vaccine-associated paralytic poliomyelitis (VAPP).
  • When the virus’s ability to jump from one person to another is restored, the result can be vaccine-derived paralytic poliomyelitis (VDPVs).

 

The inactivated polio vaccine (IPV):

  • It contains inactivated polioviruses (all three types) and is administered by injection.
  • It induces a strong systemic immune response, thus protecting against paralytic poliomyelitis, without any risk of causing VAPP or VDPV.
  • Its downside is that it hardly elicits mucosal immunity and is thus unable to prevent silent infections and transmission. So, in the event of an outbreak, only the OPV is used.
  • Also, manufacturing the IPV requires a high level of biosafety measures, because wild viruses are chemically inactivated.
  • As a result, most economically developing countries are unable to produce it locally and are forced to import it from richer countries.

 

A global switch to tackle the disadvantages of both vaccines:

  • 90% of VAPP and VDPV cases have been due to poliovirus type 2 and this virus was eradicated worldwide in 1999.
  • As a result, the OPV has attenuated versions of types 1 and 3 of the viruses (since April 2016).
  • This was accompanied by the introduction of IPV in countries that still depended on OPV in their national immunisation programmes.

 

Risks associated with this global switch: The number of VDPV cases increased after April 2016 – the cases in the US, UK and Israel were all VDPVs

 

Why did the global switch fail?

  • Limited supply/ availability of the IPV due to a sudden increase in the demand for IPV after the switch
  • Cost/ logistics
  • As the new OPV lacked the type-2 poliovirus, the population immunity against the type-2 virus dropped, giving the virus a new lease of life.
  • The interruption in routine immunisation during COVID-19 also played a part in increasing the fraction of unvaccinated and/or under-vaccinated individuals.

 

How can OPV help?

  • A child vaccinated with only the IPV will resist the virus’s ability to cause a paralytic disease, but will still be at risk of being infected, contributing to silent transmission of both vaccine-derived and wild polioviruses.
  • Hence, it would be dangerous and unethical to discontinue OPV in a hasty manner.

 

The success story of India:

  • The polio vaccination programme has successfully reduced the prevalence of polio cases by 99.9% worldwide since it was launched in 1988.
  • On 13 January 2023, India completed 12 polio-free years – a remarkable achievement that was made as a result of consistent, determined efforts and genuine commitment at all levels.
  • Since the transmission of wild poliovirus is rampant in India’s neighbouring countries (mainly Pakistan), it has persisted with pulse polio rounds that administer OPV.

 

Way ahead:

  • New alternatives: Efforts are on to develop better polio vaccines to tackle the disadvantages of both the OPV and the IPV. For example, a novel OPV (nOPV) has recently been used in African countries.
  • nOPV: It is manufactured using attenuated polioviruses in which certain mutations have been introduced using genetic engineering, making it five times harder for the virus to regain its neurovirulence.
  • Make IPV manufacturing safer: Researchers are trying to use attenuated viruses instead of wild viruses.
  • Testing specific adjuvants: That can be added to the IPV to induce a mucosal immune response.

 

Conclusion: The OPV is a champion that has succeeded in bringing the world to the brink of eradication of a dreaded disease. Until a better effective solution is available, it would be inadvisable to skip the OPV.

 

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