The story so far: A two-year-old child in Tikrikilla, Meghalaya has been infected with vaccine-derived polio. This is not a case of wild poliovirus, but an infection that presents in some people with low immunity, the Union Health Ministry said on Tuesday, August 20, 2024.
“The two-year-old child from Tikrikilla was found to have symptoms of poliomyelitis more than a week ago. The child was diagnosed with acute flaccid paralysis at a hospital in Assam’s Goalpara,” Meghalaya Chief Minister Conrad K. Sangma said. Officials in the State’s West Garo Hills district are on high alert following the confirmation of the case.
What is vaccine-derived polio?
Vaccine-derived polio 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.
OPV contains a live, attenuated virus that is used for immunisation against the disease. This weakened virus triggers an immune response when administered, thus protecting people from the disease. The attenuated virus replicates in the intestines for a limited period and is excreted in the stool. In rare cases, the virus can mutate enough to cause the disease again, and circulate in areas where either immunisation is low, or where immunocompromised persons reside, or regions with poor sanitation and hygiene. This is how vaccine-derived poliovirus (VDPV) spreads. 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.
Types of poliovirus
Polioviruses are enteroviruses that are transmitted primarily by the faecal-oral route. Three types – wild poliovirus type 1 (WPV1), wild poliovirus type 2 (WPV2) and wild poliovirus type 3 (WPV3) – have been known to exist. Symptomatically, all these strains are identical.
More about polio vaccines
The first successful polio vaccine for poliovirus was made by Jonas Salk, the director of the Virus Research Laboratory at the University of Pittsburgh, in the early 1950s. 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.
After Salk, Albert Sabin developed another vaccine that contained live polio strains, weakened by growing them serially in macaque cells, making them unfit for human infection. Since this vaccine contained the live virus, it had to be administered through its natural mode of infection – in this case, oral. This is what we today know as the OPV.
OPV is usually preferred over IPV because of its ease of administration — it does not require syringes or medical training for professionals administering it, and is inexpensive. However, the weakened virus in OPV can occasionally revert, causing the disease it is meant to prevent. IPV, on the other hand, is a less potent vaccine, but contains inactivated virus particles and hence no risk of causing vaccine-associated paralytic poliomyelitis (VAPP) – a rare, adverse reaction to OPV. IPV is comparatively tougher to manufacture too as it contains chemically inactivated virus.
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, WHO had said. WPV2 was officially declared eradicated in 2015.
However, more than 90% of vaccine-derived poliovirus outbreaks are due to the type 2 virus present in oral polio vaccines. 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.
The Indian government does not count VAPP as polio since these cases are sporadic and pose little or no threat to others, even though the number of VAPP-compatible cases showed an increasing trend in India from 1998 to 2013, as per a 2014 report in the International Journal of Infectious Diseases.
Even after the global switch from trivalent (containing all three variants) to bivalent (type 1 and type 3) oral polio vaccines in 2016 to prevent any more type 2 vaccine-derived poliovirus, the number of vaccine-derived type 2 poliovirus outbreaks have only increased sharply.
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 is less likely to cause type 2 VDPV.
