The international effort to eradicate poliovirus passed a major milestone this week with the worldwide eradication of wild strain type 3.
This announcement came Thursday, from the Global Commission for the Certification of the Eradication of Poliomyelitis just four years after eradication of wild poliovirus type 2. Leaving only a single strain, wild poliovirus type 1, on the loose.
There are three individual and immunologically-distinct wild poliovirus strains: wild poliovirus type 1 (WPV1), wild poliovirus type 2 (WPV2) and wild poliovirus type 3 (WPV3).
Symptomatically, all three strains are identical, in that they cause irreversible paralysis or even death. But there are genetic and virologic differences which make these three strains three separate viruses that must each be eradicated individually.
WPV3 is the second strain of the poliovirus to be wiped out, following the certification of the eradication of WPV2 in 2015. The last case of WPV3 was detected in northern Nigeria in 2012. Since then, the strength and reach of the eradication programme’s global surveillance system has been critical to verify that this strain is truly gone. Investments in skilled workers, innovative tools and a global network of laboratories have helped determine that no WPV3 exists anywhere in the world, apart from specimens locked in secure containment.
Type 1 is now holed up in the smallest area in the history of the disease—though that area comprises politically and geographically fraught regions of Pakistan and Afghanistan.
The current target for worldwide eradication is 2023, according to the Global Polio Eradication Initiative (GPEI), a multinational partnership that has been pursuing this goal since 1988.
Polio would be only the second human disease to be wiped out globally, after smallpox in 1980.
The 2023 target is ambitious. It would mean seeing the world’s last cases of wild poliovirus sometime next year—down from 88 cases so far in 2019—followed by a minimum of three years of intensive monitoring to certify eradication. But the antipolio effort has a recent history of success against long odds. India, where polio was paralyzing 500 to 1,000 children per day in the 1990s, eliminated the disease in 2014. The wrenching spectacle of child polio victims begging in that nation’s streets, with their twiglike legs folded beneath them, is now history.
Nigeria, where antigovernment gunmen assassinated nine women polio vaccinators in 2013, has now gone three years without any evidence of wild poliovirus—and seven years without type 3—largely through the heroic persistence of community health workers. Success there, says Carol Pandak of Rotary International, means the entire African continent could be certified free of all three strains of wild poliovirus sometime next year.
As the last two countries reporting wild poliovirus, Pakistan and Afghanistan are now feeling “tremendous global pressure to get the job done,” Pandak says. The Taliban had suspended the vaccination campaign early this year in parts of Afghanistan under its control, but last month it reversed itself and allowed polio immunizations to resume in clinics—though not in mosques or door-to-door. In neighboring Pakistan the government of Prime Minister Imran Khan recently made eradication its top priority, with Khan to assume leadership of the campaign starting next month.
But GPEI, led by the World Health Organization, the CDC, the United Nations Children’s Fund, Rotary International and the Bill & Melinda Gates Foundation, also faces daunting obstacles. The most alarming of them is the emergence and spread of vaccine-derived type 2 poliovirus, mainly in Africa. Public health workers have recognized the problem since 2000, according to Walter Orenstein, an infectious disease and polio specialist at the Emory University School of Medicine. The live but attenuated, or weakened, virus used in the Sabin oral vaccine (which is the mainstay of eradication campaigns) can sometimes spread from a vaccinated child to someone who is still susceptible to polio.
This transmission most commonly happens, Orenstein says, when a vaccine recipient is shedding the weakened virus in feces and inadvertently passes it to a susceptible person via interpersonal contact or a drinking-water source. In the subsequent chain of infection from person to person, the virus may mutate in the human body into more virulent and transmissible forms—and begin to circulate like a wild poliovirus among unvaccinated children. Last year vaccine-derived poliovirus outbreaks caused 105 cases of paralysis in children, according to GPEI.
So far, though, there has not been any way around reliance on the oral vaccine in eradication efforts, Orenstein says. The inactivated vaccine administered by injection is highly effective at protecting individuals by inducing humoral immunity (in the blood and other bodily fluids), he says, but it is not so good at inducing intestinal immunity. That makes it less useful in areas with poor sanitation, where accidentally ingesting fecal matter from drinking water is a hazard. The oral vaccine is not only easier and less costly to use in large-scale door-to-door campaigns; it also confers stronger intestinal immunity and, in the vast majority of cases, helps protect the community by reducing the amount of the virus shed in human waste.
Researchers have now identified the key points on the oral vaccine’s genome where mutations can cause it to revert to a more virulent form. That achievement has made it possible to introduce genetic modifications at those points to prevent reversion, says Jay Wenger of the Gates Foundation, which has funded the effort. Two new oral vaccine candidates with these modifications have advanced to testing for emergency-use regulatory approval by the World Health Organization and the affected countries, and they could be available as early as June. That timeline still leaves a critical eight-month window, however, in which a vaccine-derived outbreak could occur.
Another potential obstacle is fatigue at the international donor level. “We know that the last mile has proved to be the toughest phase of eradication,” says Michel Zaffran, director of polio eradication for the World Health Organization. People tend to become complacent as the success of a vaccine leads them to forget just how dreadful the disease can be. GPEI is now seeking $3.27 billion for its next four years of work, with a donor “pledging moment” scheduled for November 19 in Abu Dhabi.
“It’s always a challenge to ask for more money—especially for a disease that, for many people, no longer exists,” Zaffran says. “People ask, ‘Why is it costing so much to eradicate polio when you have so few cases left?’ But eradicating polio and stopping these outbreaks means we have to vaccinate 400 million children every year”—and then continue to vaccinate children for at least another 10 years—“to ensure there is no reemergence of the disease.” It also requires a sophisticated network to check samples from individual patients, sewage systems and open water bodies for any sign of the virus.
Fatigue at the levels of the community and individual parents could be an even greater challenge. In areas with poor sanitation, children sometimes need eight to 10 doses of vaccine to achieve immunity. Polio vaccinators have continued working to reach those children, despite recent assassinations in Pakistan. But parents naturally wonder why public health workers keep coming back to them with polio vaccine when families still lack clean water, basic sanitation or access to general health care. That situation has required the polio eradication effort to broaden its focus to other community needs and to persuade local political and religious figures to become leaders of the campaign.
Would it be more practical just to back off from the difficult goal of eradication and instead focus on merely controlling the spread of the disease? Zaffran cites a 2007 study calculating that the switch from eradication to control would cost $3.5 billion annually—and result in 200,000 cases of polio every year. “If we stop,” he says, “the disease will come back. It will rapidly spread into the Middle East, into Africa and maybe even into Europe and the United States, as we have seen with measles. I truly believe that now is the time to finish the job—and we have all the tools to do it.”
WHO
Bill and Melinda gate foundation
NHIS
Scientific American
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