Recently, the news of a two-year-old child from Meghalaya testing positive for polio has raised concern.
Vaccine Derived Polio Virus
The Union Ministry said the case was vaccine-derived and there was no need to be alarmist about it. This is not a case of wild poliovirus, but an infection that presents in some people with low immunity.
About Vaccine derived Polio Virus (VDPV)
It is a rare condition that occurs when the weakened (also called attenuated) strain of poliovirus used in the oral polio vaccine (OPV) mutates and regains the ability to cause paralysis.
- Spread Through: In rare cases, the virus can mutate enough to cause the disease again, and circulate in areas where either immunization is low, or where immunocompromised reside, or with poor sanitation and hygiene.
- Classification: According to the World Health Organization (WHO), the virus is classified as “circulating” (cVDPV2) if it is detected in at least two different sources and at least two months apart, that are genetically linked, showing evidence of transmission in the community.
- Vaccines Differentiation: More than 90% of VDPV outbreaks are due to the type 2 virus present in oral polio vaccines.
- Vaccine-associated Paralytic Poliomyelitis (VAPP) constitutes 40% of cases caused by the type 2 oral polio vaccine.
- Many cases of VAPP from type 3 virus too occur in countries using OPV.
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About Polio
Polioviruses are enteroviruses that are transmitted primarily by the faecal-oral route.
- Enterovirus is a highly contagious single-stranded RNA virus that primarily infects the intestines and is associated with several human diseases.
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- Types of Wild Poliovirus (WPV): Type 1, Type 2 and Type 3. Symptomatically, all these strains are identical.
- Type 2 Wild Poliovirus: It was declared eradicated in September 2015.
- The last detection was in India, 1999.
- Type 3 Wild Poliovirus: On World Polio Day, October 24, 2019, the WHO declared that WPV3 has been eradicated worldwide. The last case of WPV3 was detected in northern Nigeria in 2012.
- Only Type 1 wild poliovirus remains.
- Impact On: It can invade the central nervous system and as it multiplies, destroy the nerve cells that activate muscles, causing irreversible paralysis in hours.
- Polio typically affects children aged 5 years or younger.
- It can result in muscle weakness, permanent disability, and even death.
- According to the India Polio Learning Exchange (along with UNICEF): Of those paralyzed: 5-10% die when their breathing muscles become immobilized.
- No Cure But Vaccines: There is no cure for polio, but there are safe, effective vaccines which, given multiple times, protect a child for life.
About Polio Vaccines
The first successful polio vaccine for poliovirus was made by Jonas Salk in the early 1950s.
- Method Used: He grew it using a method devised by microbiologist John F. Enders and his team in 1948 to cultivate the virus in non-nerve cells.
- Salk inactivated the virus using formaldehyde and injected it into the muscles of test subjects. This inactivated polio vaccine (IPV) induced systemic immunity (relating to blood, brain, and all other organ systems) in the subjects.
- About Inactivated Poliovirus Vaccine (IPV):
- IPV protects people against all three types of poliovirus.
- IPV does not contain live virus and cannot cause disease.
- It protects people from polio disease but does not stop transmission of the virus.
- Achievement by Albert Sabin: He developed another vaccine that contained live polio strains weakened by growing them serially in macaque cells, making them unfit for human infection.
- Since it contained the live virus, it had to be administered through its natural mode of infection and was therefore administered orally. This was the Oral Polio Vaccine (OPV).
- Comparison between IPV and OPV:
- Preference: OPV is usually preferred over IPV because of its ease of administration and is inexpensive.
- Potential: The weakened virus in OPV can occasionally revert, causing the disease it is meant to prevent.
- IPV is a less potent vaccine, but contains inactivated virus particles and hence no risk of causing vaccine-associated paralytic poliomyelitis (VAPP).
- VAPP is a rare, adverse reaction to OPV. IPV is comparatively tougher to manufacture too as it contains chemically inactivated viruses.
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Rise in Cases
- In India: The Indian government does not count VAPP as polio since these cases are sporadic and pose little or no threat to others.
- However, as per a 2014 report in the International Journal of Infectious Diseases, the number of VAPP-compatible cases showed an increasing trend from 1998 to 2013.
- Around the World: After the global switch from trivalent (containing all three variants) to bivalent (type 1 and type 3) oral polio vaccine in 2016 to prevent any more type 2 vaccine-derived poliovirus, the number of vaccine-derived type 2 poliovirus outbreaks have only increased sharply.
- WHO Recommendation: The WHO authorised a genetically modified type 2 novel oral polio vaccine under Emergency Use Listing in November 2020.
- It was first used in the field in March 2021, and received WHO prequalification in December 2023.
- The vaccine is less likely to revert to neurovirulence unlike the Sabin vaccine and therefore cause less type 2 VDPV.
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22 Aug 2024
