Whenever things appear to be looking promising for the achievable goal of worldwide polio eradication, another blow seems to be dealt to efforts. Shortly after it was joyously announced that Africa had gone a whole year without a case of wild polio, an outbreak was confirmed in Mali in September, around the same time that two children in Ukraine were paralyzed by the virus – the first such cases in Europe for five years.
Now, the World Health Organization (WHO) has announced that an eight-year-old boy has died of the infection in Laos, just four days following the onset of paralysis.
While unrelated, all four cases share two things in common: poor vaccination coverage, and something called circulating vaccine-derived poliovirus (cVDPV).
You may have noticed the phrase “wild polio” in the first paragraph; this refers to one of two main types of poliovirus, the other being VDPV. Poliovirus naturally circulates in the wild, which is how people traditionally become infected. Vaccines, of which there are two types, protect against this, but one of them – the oral polio vaccine (OPV) – can, in rare circumstances, actually cause infection in others.
Unlike the inactivated polio vaccine, which consists of “killed” strains of the virus, the OPV contains live, weakened versions of the virus that are unable to cause disease, but can prompt a protective immune response. This is achieved after the vaccine-virus replicates for a limited period of the time of the gut of an immunized individual, triggering the formation of antibodies.
During this short window, the vaccine-virus also gets excreted in feces, generating an opportunity for dissemination if the region has poor sanitation. Now, this can actually be beneficial as infection with this virus can offer passive immunity to an unvaccinated individual. However, in areas with extremely low vaccination rates, the virus can linger for an extended period in communities, allowing it the chance to mutate into a form that can cause disease. This is VDPV.
This shouldn’t discourage vaccination, though; as the WHO rightly states: “The small risk of cVDPVs pales in significance to the tremendous public health benefits associated with OPV.”
In fact, we should use situations such as these tragic cases to reiterate the importance of vaccination, because it is poor coverage that allows VDPV to surface.
In this latest case, the child was from the district of Bolikhan, Bolikhamxay Province, which has persistently low vaccination rates. The percentage of individuals in this area receiving all three required doses of OPV was a meager 44% in 2015, a drop of 22% from the previous year.
Of course, being proactive is better than reactive, but regardless a prompt response to the outbreak has been initiated, with the area and its adjacent regions now being prepared for large-scale OPV campaigns. Ultimately, the aim is to phase out OPV and transition to inactivated polio vaccine so that VDPV can be eliminated, but this will not be an overnight job.
