New Delhi: At a time when the Democratic Republic of the Congo (DRC) is inching towards declaring its Ebola outbreak over, the spread of another deadly virus – polio – in the Central African country has put global eradication efforts under threat.

According to Science, despite months-long efforts of public health experts to contain the damage, the virus has continued to spread. So far, 29 children have been paralysed. Additionally, a case was recently reported outside the known outbreak zone – on the border with Uganda – which has fuelled fears of polio spreading across the continent.

A major hurdle in the path to eradication, highlighted by the current outbreak, is that a rare mutant derived from the weakened live virus in the oral polio vaccine (OPV) regains its neuro-virulence and the ability to spread. While the wild virus has been pushed close to extinction owing to OPV campaigns, Leslie Roberts writes in Science that these circulating vaccine-derived polioviruses (cVDPVs) have emerged as the greatest threat to the eradication of the virus.

In July 2016, the number of polio cases had come down by 99.9% since 1988. Nigeria, along with Pakistan and Afghanistan, were the only countries where the transmission of the virus had never been fully stopped – until new cases cropped up in the Congo as well as in Somalia last week, which seems to have further derailed eradication efforts. In order for a disease to be declared ‘eradicated’, there number of cases worldwide must be permanently reduced to zero.

Stressing the “urgency” to put a stop to these vaccine-derived outbreaks, epidemiologist Nicholas Grassly of Imperial College London said, “It is so much more important than controlling the wild virus.”

When does the weakened live virus become a threat?

OPV, being safe and effective, has for long been used for eradication efforts. But ironically, what makes OPV so powerful can also make it detrimental. For a brief period after the vaccination is administered, the weakened live virus can spread. This helps boost immunity even in those people who haven’t received the drops.

However, in some rare cases – in areas where the population is seriously under-immunised, like in the case of the Congo – the virus can continue circulating and accumulating mutations, Roberts explained.

According to the WHO, in very rare instances, the vaccine-virus can genetically change into a dangerous form that can paralyse its victims. This is what is known as a circulating vaccine-derived poliovirus. While there are three variants of the virus, the vast majority of cVDPVs are caused by serotype 2.

The problem, WHO states, is not with the vaccine but with low vaccination coverage. In cases where the population is fully immunised, they will be protected against both vaccine-derived and wild polioviruses. Even in areas with un- or under-immunised population, it takes a while for a cVDPV to occur,

Spread of cVDPVs

Since 2000, over ten billion doses of OPV have been administered to nearly three billion children worldwide. As a result, over 13 million cases of polio have been prevented.

During that time, however, 24 cVDPV outbreaks occurred in 21 countries, and almost immediately, the World Health Assembly in Geneva stated that the use of OPV must stop when the wild virus was gone.

Until 2015, over 90% of cVDPV cases were due to the type 2 component in OPV, according to the WHO. With the transmission of wild poliovirus type 2 already successfully interrupted since 1999, in April 2016 a switch was implemented from trivalent OPV – in the 155 countries still using it – to bivalent OPV in routine immunisation programmes. The removal of the type 2 component of OPV is associated with significant public health benefits, including a reduction of the risk of cases of cVDPV2.

On the flip side, however, it was clear that for a few years, some type 2 outbreaks would still occur. To fight this outbreak, a type 2 vaccine was released in ten countries and seemed to be working for the most part, except in DRC.

Outbreak in DRC

The outbreak was first detected in the Maniema province of DRC in June 2017. Just days later, another case was discovered about 900 km away in Haut-Lomami province in the southeast. By the end of the year, the number had climbed to 22.

On February 13, 2018, the Ministry of Health of the DRC declared the outbreak of circulating vaccine-derived poliovirus a national public health emergency. Swinging into action, the ministry planned reactive polio vaccination campaigns in 34 health zones deemed to be at risk: 16 in Haut-Lomami Province, 11 in Tanganyika Province, four in Haut-Katanga and three in Lomami Province, according to the WHO.

However, according to Science, owing to “remote villages, crumbling infrastructure and a weak health system,” vaccination campaigns are a challenge in DRC. The plan was to target 1,638,220 children from zero to 59 months but the campaign failed to do so.

The emergence of recent cases – especially the one near the border with Uganda – has “really increased the risk of international spread,” according to Oliver Rosenbauer, spokesperson for polio eradication at the WHO.

What next?

Mark Pallansch, a molecular virologist at the US Centres for Disease Control and Prevention, Atlanta, is of the view that if we reach a situation where type 2 spreads across the continent, the only option would be to reintroduce OPV2 into routine immunisation, which would set eradication efforts back by years.

He, however, added on a hopeful note that the spread of cVDPV2s can be managed. “I have yet to see anything that makes me think eradication is not possible. But the endgame is proving to be much more complicated than eradicating the wild virus,” Science quoted Pallansch as saying.