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Polio eradication target at risk as new cases recorded in Africa
Experts have warned that the eradication of polio is at risk because of continued outbreaks of the disease caused by the vaccine-derived virus.
The eradication of wild poliovirus is tantalisingly close - with just 19 cases of the disease so far this year in just two countries, Afghanistan and Pakistan.
The numbers have fallen dramatically over the last 30 years - from nearly half a million cases a year in the early 1980s.
However, while the numbers of wild polio are falling dramatically, there have been 45 cases of polio caused by vaccine-derived poliovirus (VDPV) in five countries this year.
VDPV occurs in countries where there is poor sanitation and low levels of vaccination coverage.
When a child is vaccinated the vaccine-virus replicates in the gut, generating an immune response. The vaccine is then excreted and very rarely can mutate and spread to other children, especially where vaccination coverage is low. This can then result in outbreaks of polio.
So far this year there have been 16 cases of polio caused by VDPV in Nigeria, 15 in both Democratic Republic of Congo and Papua New Guinea, 12 in Somalia and three in Niger - all countries with poor infrastructure and whose ability to cope with outbreaks is poor.
All of these countries - except Nigeria - have been declared as polio-free by the World Health Organization.
Nick Grassly, professor of infectious disease and vaccine epidemiology at Imperial College, London, said that if the outbreaks of VDPV were not controlled they could spread further.
“The public perception is that polio is going away but these vaccine-derived outbreaks highlight the importance of continuing to vaccinate,” he said.
Millions of doses of polio vaccine are administered every year so the risk of a vaccine-derived outbreak is still small, he said.
But he added: “The first vaccine-derived outbreak was recognised in 2000 and since that time there have been about 30 outbreaks and nearly 1,000 cases.
"Now we are getting down to very low numbers of wild poliovirus outbreaks we have to make sure these vaccine-derived outbreaks are controlled.”
2017 was the first year when there were more vaccine-derived cases than wild cases - 96 cases versus 22.
In 2016 the Global Polio Eradication Initiative switched from a vaccine containing three serotypes to one containing two serotypes in a bid to combat the spread of vaccine-derived viruses.
A recent study, led by Dr Isobel Blake, a research fellow in the department of infectious disease epidemiology at Imperial, and Prof Grassly found that the “switch”, as it became known, was a success. But while some outbreaks were expected their number and magnitude have been harder to bring to a close than anticipated.
The switch to the new vaccine means that some children have not been immunised against the serotype two polio.
“As time goes on we have a growing global cohort of kids who don’t have immunity to serotype two polio. If these outbreaks in places like DRC are not controlled the risk is that they spread further into these unvaccinated communities,” said Dr Blake.
She added: “There’s a discussion at the moment about how to certify the eradication of polio. You cannot say polio is eradicated when VDPV is in circulation.”
Professor David Salisbury, chair of the World Health Organization Global Commission for the Certification of Polio Eradication, said: “We will not have finished the job until we are also confident of two things: one is that VDPV outbreaks have been stopped and second that all polioviruses are contained.
"If you think about people doing studies in India and Pakistan who had been been collecting faecal specimens for other reasons - these specimens could have polioviruses in them. We must ensure that labs that have been handling these faecal specimens destroy them.”
He added that surveillance of polio had improved which could also account for the increase in the number of vaccine derived cases.
But he added that there was a risk of people becoming complacent as the polio eradication target date keeps falling back - it was first slated for 2000.
“The longer this drags on the harder it is for countries to maintain enthusiasm for polio interventions and the risk is that the vaccination programme doesn’t do as well we would like. The sooner we can stop polio the better,” he said.
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