The polio endgame is proving to be much more challenging with 28 children becoming paralysed by the circulating type 2 vaccine-derived polioviruses (cVDPVs) in the Democratic Republic of the Congo. It has spread to five more provinces since June 2017. And there are three different polio outbreaks in the country. Efforts to immunise all children have failed, and the outbreak has neither been controlled nor the spread prevented.
With 28 children in the Democratic Republic of the Congo becoming paralysed as of June 29, 2018 by the circulating type 2 vaccine-derived polioviruses (cVDPVs), the polio endgame is proving to be much more challenging. On July 10, 2018, the World Health Organisation assessed the overall public health risk at the “national level to be very high and the risk of international spread to be high”.
Besides the high number of cVDPVs, the outbreak, first reported from Haut Lomami Province in June 2017, has since spread to five more provinces, the latest being the Ituri Province in May this year. Since this province is close to Uganda, there is heightened risk of the virus spreading within and outside Africa. Particularly because there is known movement of people between Ituri Province and a few other countries such as Uganda, Central African Republic and South Sudan. According to the WHO, there is an increased risk of virus transmission during the imminent rainy season.
Adding one layer of complexity to the ongoing spread is the detection of three different cVDPV2 outbreaks in the DRC. While the first type 2 vaccine-derived strain reported (in June last year from Haut Lomami Province has spread to three more provinces, including the Ituri Province, two other type 2 strains have been detected from two other provinces.
With retrospective confirmation that 21 cases of acute flaccid paralysis were caused by vaccine-derived polio virus type 2, the government in February this year declared cVDPV2 to be a national public health emergency. But efforts to immunise all children have failed; the outbreak has neither been controlled nor the spread prevented.
Catch 22 situation
Thanks to intensified immunisation using oral polio vaccination (OPV), polio cases caused by the wild virus have reduced by 99.9% since 1988. However, as live, weakened viruses are used in OPV, there is a remote possibility of the virus turning virulent and causing vaccine-derived poliovirus outbreaks. This is best seen in the case of Type 2 poliovirus.
It is to eliminate the vaccine-derived type 2 polioviruses that a globally synchronised switch was made in April 2016 — from OPV containing all three strains (Type 1, Type 2 and Type 3) to only two strains (Type 1 and Type 3). Prior to the switch, an inactivated poliovirus vaccine (IPV) that uses the killed form of all three strains was introduced in a phased manner.
Despite type 2 not being used in OPV since April 2016, a few instances of type 2 outbreaks were expected to occur. The vaccine-derived type 2 capable of causing acute flaccid paralysis would have been in circulation but not been detected before. The virulent type 2 vaccine-derived strains can cause paralysis only when immunisation coverage is not high. And in the DFC, the immunisation coverage has not reached the ideal mark.
The only way to stop the vaccine-derived type 2 outbreaks is by using OPV containing only Type 2. While the renewed use of Type 2 OPV to stop the outbreak raises the risk of further shedding of Type 2 viruses, using the polio vaccine injection, which will not lead to vaccine-derived polioviruses as only killed viruses are used, is not recommended during an outbreak as it takes longer to protect the vaccinated children.