By Alfred Nyakinda

The current timeline for the global eradication of polio is under threat due to low routine immunization coverage in countries where health systems have been disrupted by conflict.

Failure to immunize children living in high risk areas and mobile populations in areas experiencing insecurity has been associated with the re-emergence of the disease in Afghanistan, Pakistan, Nigeria, Somalia and the Democratic Republic of Congo.

The 21st meeting of the Emergency Committee under the International Health Regulations (2005) (IHR) regarding the international spread of poliovirus, convened by the World Health Organisation (WHO) Director-General in May 2019 at WHO headquarters reviewed data on the wild poliovirus and circulating vaccine-derived polioviruses.

Of the three types of wild poliovirus, which is the naturally occurring form, only wild poliovirus type one is yet to be eradicated and is still circulating in Afghanistan, Nigeria and Pakistan.

The emergency committee has raised concerns about the global increase of wild poliovirus cases in 2019, especially in Pakistan where 15 cases of wild poliovirus have been reported and transmission continues to be widespread.

The committee was also concerned about attacks on vaccinators and the police protecting them in the country, as well as the refusal by communities and individuals to accept vaccination, which comes amid the resumption of the international spread of the disease between Pakistan and Afghanistan.

The danger posed by this situation has been further highlighted by the detection of wild poliovirus in sewage in Iran in an area close to Pakistan. Though no transmission has been detected in Iran and its immunization coverage remains high, this provides evidence of the danger of the spread of the disease across borders.

In addition, circulating vaccine-derived poliovirus, associated with the use of the oral polio vaccine, has also began to pose an international health risk.

The alternative Intravenous Polio Vaccine has the advantage of not resulting in circulating vaccine-derived poliovirus, but while it provides immunity to those who receive it if they are infected, they can still spread the disease through excretion.

In East Africa, low immunization coverage in Somalia has resulted in the spread of circulating vaccine-derived poliovirus type two, causing cases of paralysis. The disease has also been detected in environmental samples in Kenya, which receives large numbers of refugees from Somalia.

According to the Global Polio Eradication Initiative, the lower the population immunity, the longer vaccine-derived viruses survive and the more they mutate as they spread.

The polio virus usually affects children under the age of five who aren’t fully vaccinated, entering the body through ingestion of contaminated food and water.

If there are enough susceptible children for excreted vaccine-derived polioviruses to begin circulating, they can mutate over 12 to 18 months and become harmful, unlike the weakened virus contained in the vaccine.

Florence Kabuga of Kenya’s Ministry of Health said “We eradicated type two in the country in 2015. When we eradicated type two in the whole world, we removed the type two part from the polio vaccine. So what we’re giving is against the circulating polioviruses type one and three; that is why it’s called bivalent.”

“Last year when doing our normal routine immunizations and surveillance we isolated that type that we had eradicated in our sewage in Kamukunji. What that meant, then, is there must be someone who has come with that virus, has shed it into our environment- it could be an adult, it could be a child- and it’s circulating in our environment.”

She added that in response to the threat the ministry conducted a series of campaigns aimed at providing a monovalent vaccine to children born after 2015 who lack immunity to type two polio.

The vaccine-derived virus has also been detected in other parts of Africa, with outbreaks being reported in Nigeria, Mozambique, Niger and the Democratic Republic of Congo, while in Asia it has been detected in Papua New Guinea according to a report on polio eradication by the WHO Director General to the 72nd World Health Assembly in June.

The report noted that even though continued outbreaks continue to delay the eventual global eradication of the disease, the goal is still to eventually move from the trivalent oral polio vaccine to the bivalent vaccine to guard against the spread of vaccine-derived polioviruses.

This process began with the switch from the trivalent to bivalent vaccine between 17 April and 1 May 2016, with the aim of eventually stopping the use of the oral polio vaccine and switching to the Intravenous Polio Vaccine.

The Director General’s report went on to advise member states to minimize risks and consequences of potential vaccine-derived polioviruses by maintaining high routine immunization coverage, conducting surveillance for any emergence of circulating vaccine-derived poliovirus and maintaining strong outbreak response capacity.

The Global Polio Eradication Initiative notes that vaccine-derived polioviruses appear to be less transmissible than wild poliovirus, with outbreaks usually being self-limiting or rapidly stopped with two to three rounds of high-quality supplementary immunization services.