Another milestone on the long and costly, sometimes discouraging, road to polio eradication: Global Health officials have announced Wednesday two of the three strains of wild polio virus have been officially eliminated.
Although it still brings the world closer to eradication, the effort took a lot longer than expected. When the campaign began in 1988, most public health officials and donors expected the battle to be over by the year 2000.
But two major obstacles have emerged.
Secondly, in some countries the viruses used in the oral vaccine have mutated into a form that can be transmitted through diapers and sewage and can paralyze unvaccinated children. This has contributed to the fear of the oral vaccine, even though complete vaccination is the only protection against these mutant viruses.
Just in the last two months, cases of paralysis caused by mutant vaccine viruses have been reported in the Philippines, Zambia, Togo and Chad. As paralysis only occurs in about one in 200 polio cases, experts speculate that far more children have been infected.
To end such epidemics, it is usually necessary to vaccinate hundreds of thousands of children at a time with the injectable vaccine, which contains the killed virus, which can not mutate, and with the oral vaccine. The latter contains weakened viruses that can not normally cause disease, but offer better protection than killed viruses.
The strain that the Global Certification Commission for Poliovirus Eradication declared eliminated this week is the wild type 3 polio virus, of which the last case was seen in Nigeria in 2012. Type 2 was declared eliminated in 2015; the last case was detected in India in 1999.
Type 1, the only remaining wild strain, circulates only in Pakistan and Afghanistan.
(In the 1950s, the three strains bore more evocative names: Brunhilde, Lansing and Leon.The first was named in honor of a laboratory chimpanzee, the second after the city of Michigan where she was isolated and the third after the death of a boy from Los Angeles. The nicknames then fell out of favor.)
Huge multi-year monitoring efforts are needed before a viral strain can be declared extinct. Children can be paralyzed by many other viruses, bacterial infections of the brain, and neck and spine injuries.
To make sure that polio is not the cause, stool samples must be taken every year from more than 100,000 paralyzed children. Thousands of samples of sewage and water are collected in 70 countries; the virus can be detected at concentrations of parts per million.
"The certification board has been very, very careful," said Dr. Walter A. Orenstein, polio expert at the Emory Vaccine Center in Atlanta and former director of immunization for the Centers for Disease Control and Prevention.
Over the past decade, a new dangerous front has opened up in the polio war.
In countries where vaccination rates are low, weakened viruses in the oral vaccine can circulate in wastewater and become perverse twins.
Stacking at random genetic changes, or exchanging genes with other intestinal viruses like the Coxsackie virus, the viruses can become virulent and paralyze children who have never been vaccinated.
In the past two years, outbreaks of cVDPV – which means "circulating polio virus derived from a vaccine" – have affected nearly 20 countries.
Although most of these epidemics have been minimal and have finally been brought under control, more and more children are now paralyzed by cVDPV compared to type 1 in Pakistan and Afghanistan.
For example, up & # 39; here this year, 88 Pakistani and Afghan children were paralyzed by the last wild strain, while 95 children from Africa and Asia were paralyzed by virus derived from vaccines.
To avoid this, the eradication campaign takes several steps.
First, health officials try to ensure that every child in the world receives at least one dose of the injected vaccine. It circulates in the blood, so that a child can still get an infection of the bowel but not be paralyzed.
Secondly, one year after the elimination of type 2 polio worldwide, the campaign introduced a new "bivalent" vaccine without type 2 vaccine. weakened virus.
But there will be no "monovalent" vaccine with only the weakened type 1 virus, said Michel Zaffran, director of poliomyelitis eradication at the World Health Organization.
"Type 2 was so powerful that he dominated the old vaccine," he said. "Deleting type 3 will not make the current more immunogenic."
It was a bureaucratic nightmare, he added, to get all the countries of the world to import and refrigerate hundreds of millions of new doses of vaccine and to safely destroy old ones.
"We do not need to create a new problem," Dr. Said Zaffran.
Third, the Bill and Melinda Gates Foundation supports the creation of new oral vaccines less able to mutate into dangerous forms.
"Tighten the ends" by cutting some nucleotides The portion of the genome that acts as gatekeeper leaves it less likely to exchange genes with other intestinal viruses, said Dr. Ananda S. Bandyopadhyay, a polio program officer at the foundation.
In addition, rcreate the genes creating polymerase, which helps the virus copy itself, means fewer potentially dangerous copy errors.
As most recent outbreaks were caused by mutant versions of type 2, the foundation has launched accelerated clinical trials on this strain of the new vaccine, said Dr. Bandyopadhyay.
"If everything goes well, it could be ready by 2020," he said.
New versions of type 1 and type 3 vaccines are expected to follow in two years, he said.
The new versions are not intended for routine immunization, he said, but an emergency stock used to combat epidemics.