ISLAMABAD: After the United Kingdom discovered the poliovirus in sewage samples in London for the first time in the last four decades, suspicion has fallen on Pakistan as being the source.

However, health authorities in Islamabad claim the “vaccine-derived virus” found in the UK exists in 22 countries and the type found locally was the wild poliovirus (WPV).

The UK Health Security Agency has noted that the virus was probably imported from some country, and has asked parents to ensure their children were fully immunised against the crippling disease.

Pakistan Polio Eradication Programme National Coordinator Dr Shahzad Baig told Dawn on Thursday that it was too early to say the virus had travelled from Pakistan, as UK authorities were yet to announce the results of genome sequencing.

Official says genome sequencing report will ascertain origin of virus

Genome sequencing helps determine the origin of a virus, as samples detected in different areas have a different ribonucleic acid (RNA). A number of times, cases reported in Pakistan were reported to have originated from the neighbouring Afghanistan during genome sequencing.

“We are waiting for the genetic sampling report of the virus. Moreover, there are two types of polioviruses in the world: WPV that is present in Pakistan and Afghanistan, and the vaccine-derived poliovirus (VDPV). The environmental samples in London were found to be of the VDPV, which exists in 22 countries, so Pakistan should not be blamed for exporting it before getting the report of genetic sequencing,” Dr Baig stressed.

He said the vaccine-derived variety could have travelled from Nigeria, Egypt, Pakistan, Afghanistan, Yemen, Sudan, Central Africa and a number of other countries.

An expert from the Ministry of National Health Services, who was not authorised to speak on record, said there were three types of WPV. “Decades ago, a vaccine named ‘Trivalent’ was used as it contained all the three types. After the eradication of type II in 2016, a ‘Bivalent’ vaccine was introduced with types I and III of the WPV. However, in 2020, the type II remerged suddenly due to which it was feared that the virus had the potential to resurface. This second type of the virus is called the VDPV, as it was totally eradicated from the world,” he maintained.

“The oral polio vaccine (OPV) contains an attenuated (weakened) vaccine virus, activating an immune response in the human body. When a child is administered the OPV, the weakened vaccine virus replicates (grows) in the intestine for a limited period, thereby developing immunity by building up antibodies. During this time, the vaccine virus is also excreted. In areas with inadequate sanitation, this excreted vaccine virus can spread within the immediate community (and offer protection to other children through ‘passive’ immunisation), before eventually dying out,” he added.

“On rare occasions, if a population is seriously under-immunised, an excreted vaccine virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. In very rare instances, the vaccine virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus,” the expert explained.

Dr Nadeem Jan, a public health expert, told Dawn Pakistan will be in a difficult, rather embarrassing, situation if it was proved that the virus found in London travelled from here.

“Pakistan has been already under a travel advisory due to which every person has to get vaccinated and carry a vaccination certificate before travelling abroad. We also claim that over 90 per cent vaccination target has been achieved. The world may doubt our claims if it is found that the virus belongs to Pakistan,” he remarked.

Dr Shahzad Baig, while replying to a question, claimed the poliovirus was generally in control in Pakistan, except North Waziristan where there were refusals towards the vaccine and people were not cooperating with polio teams.

“Around 200 countries have already eradicated the virus, and Pakistan can also achieve the target. Unfortunately, North Waziristan has become a safe haven for the virus, but if we are able to defeat the virus there, we will be able to eradicate it from the country,” he stressed.

Published in Dawn, June 24th, 2022