Vaccine-Related Polio Outbreak in Nigeria Raises Concerns

Leslie Roberts

Northern Nigeria has been hit by one of the largest known outbreaks of poliomyelitis caused by the live polio vaccine itself. The ongoing outbreak could be a serious setback for the global polio eradication campaign: It is occurring in a region where rumors about vaccine safety derailed vaccination efforts several years ago.

Experts with the Global Polio Eradication Initiative emphasize that the widely used trivalent oral polio vaccine (OPV) is safe. But the low immunization rates in northern Nigeria have created the conditions for the attenuated vaccine virus to regain its virulence and trigger an outbreak.

Detected in September 2006, the outbreak of vaccine-derived poliovirus (VDPV) type 2 was immediately reported to the World Health Organization and Nigerian health officials. But the information is just now being released publicly--in the 28 September Morbidity and Mortality Weekly Report and WHO's Weekly Epidemiological Record--a delay that has caused some consternation in the polio community. Officials say they were worried that the news, if misconstrued, could again disrupt polio vaccination efforts in Nigeria.

"There were legitimate concerns that anti-polio vaccination rumors would be rekindled by an incomplete explanation of the cause of the VDPV outbreak," says Olen Kew, who has led efforts to analyze the outbreak from the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia.

Double whammy. Northern Nigeria is battling wild and vaccine-derived poliovirus. CREDIT: JEAN-MARC GIBOUX

Several polio experts told Science that although they understand how sensitive the situation is, they disagree with the decision to keep quiet. "I am troubled that the information hasn't come out, absolutely," says Donald A. Henderson of the University of Pittsburgh Center for Biosecurity in Baltimore, Maryland. Henderson says details of each outbreak are essential if scientists are to understand just how risky these vaccine-derived strains are.

So far, there are 69 confirmed cases of paralysis, and more suspected, caused by VDPV in nine northern Nigeria states, says Kew. The case count seems certain to rise. About half the cases have occurred around Kano, a largely Muslim state where anti-Western sentiment and rumors that the vaccine caused sterility or AIDS led several states to halt polio vaccination in 2003. After repeated demonstrations of the vaccine's safety and considerable behind-the-scenes diplomacy, vaccinations resumed about a year later, but the damage had already been done.

By the end of 2004, the number of polio cases in Nigeria had doubled to about 800, and in 2006 it soared to more than 1100. Wild virus from Nigeria reinfected some 20 other countries, leading to a spike in global cases. It was a huge setback to the Global Polio Eradication Initiative, which estimates that the world spent an additional $500 million to contain the damage. Only recently have global cases dropped back to near preboycott levels.

Although Nigeria has since made considerable progress, wild poliovirus, both type 1 and type 3, is still circulating in the north, and vaccine coverage there remains low. In 2006, between 6% and 30% of children in the north had never received a single dose of OPV.

Those are exactly the conditions that render an area susceptible to outbreaks of vaccinederived virus. Since the 1960s, scientists have known that attenuated viruses can in rare instances mutate and regain virulence, but it was only in 2000, with an outbreak in Hispaniola, that they realized VDPVs could spread disease from person to person.

The current outbreak came to light when a technician at the CDC polio lab noticed a preponderance of type 2 virus in the isolates sent in from northern Nigeria. That instantly raised suspicion, Kew says, because wild type 2 poliovirus has been eradicated globally. That meant the only possible source was the trivalent vaccine, which had been used in Nigeria in preboycott campaigns. Since Nigeria resumed vaccinations in 2004, says Kew, it had "quite properly" been using the more effective monovalent vaccines against wild types 1 and 3 in its campaigns. Genetic analysis quickly confirmed the source; it also suggests that several VDPVs emerged independently in 2005 and 2006, multiple times.

In earlier outbreaks, circulating VDPVs have been relatively easy to stamp out, but this one has persisted despite four campaigns with trivalent OPV in the past year. "We suspect it is simply because the coverage was not adequate; we don't believe there is anything exceptional about this virus," says Kew. As evidence, he notes that two VDPV strains jumped from Nigeria to Niger, where routine vaccination is almost 90%. Both "barely made it 5 kilometers before they dead-ended," he says.

Polio expert Oyewale Tomori, vice chancellor of Redeemer's University near Lagos and chair of Nigeria's expert advisory committee for polio eradication, says he has been urging officials to go public. He worries that secrecy might fuel suspicions about vaccine safety instead of reinforcing the need to intensify immunizations in Nigeria.