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Pakistan’s seven new polio cases explained

November 13, 2019
Pakistan’s seven new polio cases explained

Photo: Online

The discovery of seven fresh polio cases in Pakistan has prompted the government to plan vaccinations from November 18. These new cases became controversial after The Guardian published a story alleging they were covered up and that the children had caught a type of polio that had been globally finished off.

SAMAA Digital spoke to the chief epidemiologist who has been brought back to run the government’s vaccination campaigns. We asked national coordinator for polio eradication Dr Rana Safdar about Nov 7 The Guardian story and whether he agreed with its characterisation of the situation.

The cases

Between August and October this year, seven cases of polio were detected in Khyber Pakhtunkhwa’s districts of Kohistan and Torghar and Gilgit Baltistan’s Diamer district.

There are three types of Wild Polio Virus: 1, 2 and 3. Two and three have been globally eliminated, according to the WHO. But The Guardian reported that 2 had returned. Dr Safdar refuted this.

He said that the children did not contract the eliminated old type 2.

More accurately, theirs were cases of polio contracted from a mutated version of the virus in the environment. They got what is called vaccine-derived polio. It is called vaccine-derived virus type 2 or VDPV2.

Explainer

This is considerably confusing, so we thought we would break down the basics.

If you have a virus you need to vaccinate people against it. The vaccination itself is made from a weak strain of the virus. You give the vaccine to children and so their body develops the immunity to it.

When we vaccinate children in Pakistan against polio, we give them the Oral Polio Vaccine (OPV) drops. This is a live but weak or attenuated form of the virus. In reaction, the children develop antibodies that protect them against active infection.

However, sometimes because the OPV is live, a child will get the drops, become immunised (and safe) but excrete a weak strain of the virus from the OPV and introduce it to the environment. This is what they call vaccine-derived polio virus or VDPV.

This kind of thing happens rarely. But just to be safe, governments keep frequently vaccinating children. However, if you leave too long a gap between campaigns, there is a risk of this happening.

In these recent seven cases, Dr Safdar says the children have VDPV. More specifically, VDPV is a mutated form of the Sabin strain of polio virus that spreads in under-immunised areas after the vaccine virus is excreted and ends up in the environment, according to the WHO.

For cases of paralysis to occur, the mutated virus needs to have been circulating in vulnerable populations for at least 12 months. This is why the WHO and Global Polio Eradication Initiative say that gaps in immunisation campaigns are the main reason why VDPV spreads.

What the government is doing

The new immunisation campaign will be carried out in eleven KP districts, including Abbottabad, Mansehra, Torghar, Kohistan and Shangla. It will target around 2.4 million children against the recently emerged strain.

A similar campaign started Monday in Rawalpindi and Diamer and other districts of Gilgit Baltistan.

“We are administering a special vaccine for polio virus type 2 for the children living in those areas,” Dr Rana Safdar said.

When the government detected the seven cases, it developed a two-pronged strategy. The first line of action was to mitigate risks and protect vulnerable children. “In the outbreak areas around 180,000 children were vaccinated with Inactivated Polio Vaccine,” said Dr Safdar. The IPV, or inactivated polio vaccine, is the injectable form of the vaccine which provides immunity against all three types of polio virus.

The Global Polio Eradication Initiative says IPV is highly effective in preventing paralytic disease caused by all three types of polio virus; however, it does not stop the transmission of the polio virus in a community. This is why OPV, the oral polio vaccine we use, is the vaccine of choice.

The government’s next step was to investigate if any old strain of the polio vaccine was used during any of the campaigns.

In April 2016, there was a global switch from the trivalent to bivalent OPV. The component that acted against type 2 wild polio virus was withdrawn. All old vaccines were supposed to be discarded.

“A very extensive search was carried out to see if the old vaccine was used,” said Dr Safdar. “Every facility was examined individually but there was no indication of the switched vaccine being used.”

The virus isolates were then sent for genetic sequencing. Last week, it was confirmed they were of vaccine-derived polio virus type 2 (VDPV2), Dr Safdar said.

There are around 20 countries in the world where there have been VDPV outbreaks. Even now, the outbreak is ongoing in more than a dozen countries: Nigeria, Kenya, surrounding countries in Africa and Philippines and China in Asia. These have mostly been the result of children’s immunity to the virus strain going down after the global switch.

This year, Pakistan has recorded 82 cases of polio. However, these have all been WPV1, Dr Safdar emphasised, adding that we have eradicated type 2 and 3.

The last case of type 2 was seen in 1999 and the last case due to infection by type 3 was reported in 2012.

The unanswered questions

It is not clear if a gap in immunisation is the sole reason for these seven new cases. Dr Safdar said that they were investigating if the virus was brought in to Pakistan by travellers from areas where there is an active outbreak. What this theory would not explain, however, is why the children would have contracted it had they been immune.

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