The ongoing outbreak in northern Nigeria, which started in 2005, is also responsible for two cases of polio in neighbouring Niger, the scientists reported in Morbidity and Mortality Weekly Review, a journal published by the U.S. Centers for Disease Control.
The article details polio cases that trace back to circulating vaccine viruses, one of the major wild-cards in the labouring international effort to eradicate polio.
Such vaccine-linked outbreaks have occurred before. But this one has the potential to pose a serious threat to the polio eradication effort because of its location. Several states in Northern Nigeria suspended polio immunization efforts for nearly a year in 2003-04 in response to vaccine safety rumours that led parents to refuse to have their children immunized.
"Given the situation in Nigeria antecedent to where we are, this could be frightening," Dr. Oyewale Tomori, vice-chancellor of Redeemer's University in Redemption City, Nigeria, admitted in an interview at a recent international polio symposium in Washington, where the cases were discussed.
Tomori, a former World Health Organization official, said if information on the cases isn't conveyed in context in Nigeria, "it could set the immunization program backwards."
The context he referred to is the fact that vaccine-derived polio virus cases - known in the shorthand of polio as VDPVs - are the result of low immunization rates in the places where they occur.
"VDPVs have occurred in many countries. And they occur when (vaccine) coverage is low. And this is a reflection of low vaccination coverage of Type 2 vaccine in Nigeria," Dr. David Heymann, the WHO official who heads the polio eradication program, said from Geneva.
The polio eradication program is a partnership between the WHO, the CDC, UNICEF and Rotary International. Begun in 1988, the program has spent more than US$5.3 billion in the so-far elusive bid to eliminate polio, a formerly ubiquitous virus that causes crippling disease in one out of every 200 people it infects.
While they acknowledge the sensitivity of the topic, a number of polio experts are dismayed the outbreak has taken so long to come to light. They've been hearing about the cases for months, through contacts and back channels.
"This has been going on for more than a year and a half and we have nothing at all about it until now? If we're this concerned about the VDPVs, let the information become public," Dr. D.A. Henderson, the infectious diseases expert who led the successful smallpox eradication program, said from Baltimore, Md.
Expert opinion is divided over the danger posed by VDPVs. The WHO insists the transmission chains formed by these viruses are easier to break than those created by wild polio viruses. Others believe these viruses, if left unchecked, will become every bit as dangerous as wild polio viruses.
The report details vaccine-derived outbreaks or individual cases in Cambodia, Myanmar, China, Iran, Syria, Kuwait and Egypt. But each comprised fewer than a handful of cases. By contrast, the Nigerian outbreak is the largest on record involving vaccine-derived viruses.
The northern Nigeria boycott led to an explosion of polio cases in the country, one of only four still on the WHO's list of nations in which the virus remains endemic. The fallout continues to besiege eradication efforts, both because of ongoing transmission in Nigeria and cases exported to multiple other countries around the globe.
Polio experts say the suspension actually seeded the current outbreak, which involves Type 2 polio vaccine viruses. (There are three strains of wild polio virus, numbered 1 through 3. Ironically, wild Type 2 polio viruses haven't been seen since 1999 and are believed to have been eradicated.)
"We do not see this in southern Nigeria. We do not even see this in Niger. It got just across the border in Niger, two independent importations, two cases, right along the border. It didn't go any further," said Dr. Olen Kew, a polio expert at the CDC in Atlanta, where polio viruses are typed and traced for the eradication program.
"So the population immunity conditions in northern Nigeria are different than in the surrounding areas."
Oral polio vaccine is no longer used in Canada but it is the vaccination tool of developing countries, because of its low cost and ease of administration. The oral drops don't require needles (or safe needle disposal) and don't need to be delivered by health-care professionals.
The vaccine contains live, weakened viruses that stimulate immunity by causing low-level infection in children who receive the drops. And while effective enough to wipe out polio in over 100 countries, it comes with a couple of significant risks.
On rare occasions oral polio vaccine paralyzes children. About one out of every one million doses leads to paralysis.
And the vaccinated children shed viruses in their stools for weeks. Those viruses mutate. If they circulate long enough, the built-up mutations can restore the virulence stripped out in the vaccine production process, giving these viruses back the power to paralyze months and even years after their progenitors came out of a vaccine vial.
It is thought vaccine-derived viruses don't get a chance to circulate long enough to regain virulence in countries where vaccination rates are high and most children have immunity to polio.
Tomori said it is critical that Nigerians understand that it is under-vaccination - not vaccination itself - that caused the problem.
"It is important for people to know that these are the factors, these are the reasons, this is what happens," he said, insisting his country will succeed in ending spread of polio within its borders.
"I have to believe that. There's no other way."
