What is a vaccine-derived polio virus and why is it a threat?

In a vaccine-derived polio virus (VDPV), the source of the virus is the vaccine itself. Here are five things to know about VDPV and the concerns around it

The Telangana government on Tuesday called for a special polio immunisation drive in Hyderabad, as part of preventive action after a surveillance study conducted by the World Health Organisation (WHO) found traces of vaccine-derived polio virus in the city’s Amberpet nala, a major sewage canal in the city.

Here are five things to know about vaccine-derived polio virus (VDPV) and the concerns around it.

What is polio?

Poliomyelitis or polio is a highly infectious disease caused by a virus. It invades the nervous system, and causes paralysis, medically known as an acute flaccid paralysis (AFP), which is characterised by sudden muscle weakness and pain in the limbs. The disease is transmitted from person to person, mainly through the faecal-oral route, affecting children under five years of age. In the absence of wild polio virus (WPV) transmission, India was declared a polio-free country in March 2014, after years of relentless vaccination.

What does the Hyderabad study reveal?

For maintenance of polio eradication in polio-free countries, WHO conducts surveillance for cases of AFP and collects samples of sewage water to find any traces of polio viruses. In one such study in Hyderabad in April, out of 30 samples collected, one sample from Amberpet nala contained traces of type-2 VDPV. Lab tests have revealed that the virus has passed through human body and has undergone mutation or nucleotide change. Similar virus strains were detected in Delhi, Bihar and Gujarat. The Telangana government asked people not to panic, and called for a one-week special polio immunisation drive in Hyderabad, starting from 20 June, to vaccinate around 300,000 children.

What is a vaccine-derived polio virus?

In VDPV, the source of the virus is the vaccine itself. The oral polio vaccine called polio drops, which India deployed extensively to fight against polio, contain a live, attenuated or weakened polio virus. When a child is vaccinated, the weakened vaccine-virus replicates in the intestine and enters into the bloodstream, triggering a protective immune response in the child. Like wild poliovirus, the child excretes the vaccine-virus for a period of six to eight weeks. Importantly, as it is excreted, some of the vaccine-virus may no longer be the same as the original vaccine-virus as it gets genetically altered during replication. In areas of inadequate sanitation, this excreted vaccine-virus can quickly spread in the community and infect children with low immunity.

Why is VDPV a matter of concern?

The cases of paralysis due to VDPV are rare as the virus has to circulate for a long time in the community of under-immunised population before it can infect and cause paralysis in someone. Vaccine-associated paralytic poliomyelitis (VAPP) occurs in an estimated 1 in 2.7 million children receiving their first dose of oral polio vaccine, according to the Global Polio Eradication Initiative, a public-private partnership of national governments and WHO.

The aspect that is a matter of concern is that India reports high number of non-polio—AFP or paralytic—cases in children who are less than 15 years of age, which the study links to the VDPV. According to WHO, more than 50,000 AFP cases are investigated in India every year as a part of its surveillance system.

OPV vs IPV

The detection of VDPV in Hyderabad and other places has intensified the discussion on replacing oral polio vaccine (OPV) with inactivated polio vaccine (IPV). IPV given through an injection contains inactivated virus, considered to be safer than OPV that contains live virus. WHO has been advocating IPV over OPV as part of its global endgame strategy on polio eradication. India introduced IPV in the mandatory immunization programme on 1 December in six states. For the time being, IPV will be given in addition to the existing OPV. OPV has its strong advocates who believe that the vaccine is best suited for countries such as India due to its low cost, high efficacy and ease of administering, and argue that the safety concerns are overstated compared to the benefits of the vaccine.