Virologist Vincent Racaniello breaks down the first case of polio in the US in nearly a decade.
Today, on World Polio Day, wild poliovirus type 3 has been declared eradicated by a commission of the World Health Organization. The last case of type 3 poliomyelitis was recorded in 2012 in Nigeria. Because wild poliovirus type 2 was declared eradicated in 2015, now only wild poliovirus type 1 continues to circulate, causing paralysis in Afghanistan and Pakistan.
Wondering why the eradication certificate (pictured) says ‘wild’ poliovirus, and not simply poliovirus? The reason is that bivalent oral poliovirus vaccine, containing types 1 and 3, continues to be used globally. After oral administration of this vaccine, vaccine-derived strains are excreted in the feces. Although wild poliovirus type 2 was declared eradicated in 2015, vaccine-derived type 2 poliovirus strains continue to circulate. These vaccine-derived viruses have so far in 2019 caused 102 cases of poliomyelitis.
In other words, we will not be able to declare that poliovirus is eradicated until we stop using the oral poliovirus vaccines. Use of the type 2 oral poliovirus vaccine was stopped in 2016, and WHO suggested that at least one dose of inactivated poliovirus vaccine (IPV) – containing all three serotypes – be included in immunization schedules. Lower IPV use than anticipated has led to continued circulation of vaccine-derived type 2 poliovirus. Compounding the problem is the use of OPV to control outbreaks of vaccine-derived paralysis, leading to introduction of more vaccine viruses into the environment.
Vaccine-derived type 1 poliomyelitis is quite rare, and so if we can vaccinate properly in Afghanistan and Pakistan, we will likely be able to eradicate this serotype. The solution to the problem of type 2 poliovirus will require a complete global switch to IPV, or the use of a new vaccine that cannot revert during replication in the gut and cause paralysis. Such vaccines are in development.
The TWiV team covers outbreaks of eastern equine encephalitis virus in the US and poliovirus in the Philippines, and explain how a chemokine induced by HIV-1 infection helps release more virus particles from cells.
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The global withdrawal of the Sabin type 2 poliovirus vaccine is a test of the feasibility of the plan, declared by the World Health Assembly in 1988, to eradicate all polioviruses.
The TWiV team reviews the first FDA approved gene therapy, accidental exposure to poliovirus type 2 in a manufacturing plant, and production of a candidate poliovirus vaccine in plants.
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I cannot let September pass without noting that 34 years ago this month, I arrived at Columbia University to start my laboratory toÂ do research on poliovirus (pictured). That virus is no longer the sole object of our attention – we are wrapping up some work on poliovirus andÂ our attention has shifted elsewhere. But this is a good month to think about the status of the poliovirus eradication effort.
So far this year 26Â cases of poliomyelitis have been recorded – 23 caused by wild type virus, and three caused by vaccine-derived virus. At the same time in 2015Â there wereÂ 44 reported cases of polio – smallÂ progress, but, in the words of Bill Gates, the last one percent isÂ the hardest.
One of the disappointmentsÂ this year is Nigeria. It was on the verge of being polio-free for one year – the last case of type 1 poliovirus in Nigeria had beenÂ recorded in July of 2014. In AugustÂ the government reported that 2 children developed polio in the Borno State. The genome sequence of the virus revealed that it had been circulating undetected in this region since 2011.Â Due to threats from militant extremists, it has not been possible for vaccination teams to properly cover this area, and surveillance for polioviruses has also been inefficient. The virus can circulate freely in a poorly immunized population, and as only 1% of infections lead to paralysis, cases of polio might have been missed.
The conclusion from this incident is thatÂ the declaration that poliovirus is no longer present in any region is only as good as the surveillance for the virus, which can never be perfect as all sources of infection cannot be covered.
Of the 26 cases of polio recorded so far in 2016, most have been in Afghanistan and Pakistan (9 and 14, respectively). It is quite clear that conflict has prevented vaccination teams from immunizing the population: in Pakistan, militants haveÂ attacked polio teams during vaccinationÂ campaigns.
Recently 5 of 27 sewageÂ samples taken from different parts of the province of Balochistan in Pakistan have tested positive for poliovirus. Nucleotide sequence analysis revealed that the viruses originated in Afghanistan. The fact that such viruses are present in sewage means that there are still individuals without intestinal immunity to poliovirus in these regions. In response to this finding, a massive polio immunization campaign was planned for the end of September in Pakistan. This effort would involve 6000 teams to reach 2.4 million children. Apparently police will be deployed to protect immunization teams (source: ProMedMail).
The success of the polio eradication program so far has made it clear that if vaccines can be deployed, circulation of the virus can be curtailed. If immunization could proceed unfettered, I suspect the virus would be gone in five years. But can anyone predictÂ whether it will be possible to curtail the violence in Pakistan, Afghanistan, and Nigeria that has limited polio vaccination efforts?