The new figure is double the 62 vaccine-derived cases reported in the country last year, and marks the continuation of the longest vaccine-derived polio outbreak ever seen, beginning in 2006, according to Oliver Rosenbauer, spokesman for World Health Organization’s Polio Eradication Initiative.
The oral vaccine contains weakened live polio virus to build immunity in the patient, but can spread from a vaccinated person to the unvaccinated public and grow more virulent. If it is not contained and is allowed to mutate for months, it can grow strong enough to cause paralysis.
“Where you get vaccine-derived cases is in communities where the routine vaccination is very low,” said Robert Scott, chair of the International PolioPlus Committee of the Rotary Foundation. Rotary International, the WHO, and the Centers for Disease Control and Prevention are part of a consortium of groups that support Nigeria’s immunization drive.
“In this particular case in Nigeria, they stopped vaccinating for almost a year, so large areas were unvaccinated,” Scott said.
Nigeria halted its polio vaccination program for more than a year in 2003 because of rumors the vaccine was a conspiracy against Muslims and that it caused infertility. With the encouragement of international health organizations, the country has worked to improve its coverage and implementation of polio vaccinations in recent years, but many remain unvaccinated.
Vaccine-derived cases of polio have been seen in 12 countries with a total of 383 cases reported worldwide over the last 10 years, according to Rotary figures. In that same period, some 10 billion doses of the vaccine were given to children, according to Scott.
The oral vaccine containing the live virus is used in Nigeria because it is the only vaccine that can eradicate the active disease, WHO’s Rosenbauer explained.
Many countries, including the United States and Canada, used the oral vaccine containing live virus to eradicate the disease, then were able to switch to an injected vaccine containing dead virus once polio was largely eliminated among the population.
The oral vaccine also has other advantages; it is cheap and can be administered by anyone. While the live virus vaccine means that the vaccinated patient can pass it to those around them, in most cases that can also be an added benefit, building immunity in those who have not received the vaccine, through a process called passive immunization.
But when the unvaccinated population is as large as Nigeria’s, the virus continues to spread and mutate.
There are three types of poliovirus. The “wild”, or naturally occurring, form of type 2 polio virus was eliminated in 1999, but type 1 and type 3 continue to circulate.
Tackling the vaccine-derived, type 2 outbreak in Nigeria was complicated by a 2008 outbreak of type 1 polio. It prompted an emergency response with a vaccine that targeted just that type, allowing the vaccine-derived strain to spread unchecked.
“This year, yes we’ve seen an increase in vaccine-derived cases but the flip side is that the type 1 [polio], which is the much more dangerous strain, they really hit that over the head,” said Rosenbauer.
Even with the vaccine-derived outbreak, overall cases of polio in Nigeria are down from 575 by August 2008 to 363 the same time in 2009, reported Rotary.
After the vaccine-derived outbreak was detected in 2006, Nigeria’s vaccination program distributed vaccines that target multiple strains of polio, but the effort was not enough to stop the outbreak. In 2009, two rounds of vaccines that cover all three types have already been administered, and groups working with the campaign hope the vaccine-derived outbreak will be contained.
Both Rosenbauer and Scott agree political, religious and community leaders are now highly involved in the polio vaccine programs in Nigeria, and both said they do not believe the vaccine-derived causes will revive the rumors, or have a detrimental effect on the vaccine campaign.
“The level of community engagement is completely different that it was five years ago” said Rosenbauer.
There are fears, however, that the vaccine-derived outbreak could revive the spread of type 2 polio. At this stage, the mutated virus causing the vaccine-derived cases still does not have the virulence of the “wild” type, Rosenbauer said. However, if the strain is not contained and continues to spread and mutate, it is unknown how strong it could grow.
Polio has been eradicated in all but four countries in the world: Afghanistan, Pakistan, India and Nigeria. Scott said he is encouraged by the drop in cases in Nigeria and said it is a step towards eradication in the country.
“We’ve done it in 99.9 percent of the world, in countries where we had equal challenges,” Scott said. “It is a question of getting the vaccine into the mouths of the children.”