WHO mulls phase out of vaccine linked to polio outbreaks.

(WHO) is contemplating discontinuing the use of an oral polio vaccine that’s been linked to ongoing outbreaks of the virus in the developing world. WHO is weighing the possibility of using a different formulation of the vaccine, but maintains that patchy implementation of immunization programs, not the vaccine itself, is to blame for the outbreaks. Since 2000, some two billion children have received more than 10 billion doses of oral polio vaccine containing up to three live strains of the virus, says Oliver Rosenbauer, spokesperson for the WHO’s Global Polio Eradication Initiative. Nineteen outbreaks of circulating vaccine-derived poliovirus (cVDPV) have been documented in that same period, resulting in 536 cases in some 17 countries, the majority in Africa, Rosenbauer says. One outbreak in Nigeria has been ongoing since 2005, with 13 new cases reported so far this year (www .polio eradication.org/Dataandmonitoring /Po lio thisweek/Circulatingvaccine derived poliovirus.aspx). While less virulent than wild poliovirus, and therefore more easily controlled, cVDPV is just as harmful, Rosenbauer says. “Once wild poliovirus eradication has been achieved, the use of oral polio vaccine in routine immunization will need to be stopped, and work is already underway to prepare for this.” Outbreaks of cVDPV occur when routine immunization activities are poorly conducted and a population is left susceptible to poliovirus, whether wild or vaccine-derived, Rosenbauer explains. “The problem is not the vaccine itself, but low vaccination coverage. If a population is fully immunized, they will be protected against both vaccine-derived and wild polioviruses.” More than 90% of all cVDPV cases stem from the mutation of just one of the three live polio strains in the vaccine, type 2 vaccine-virus, says Rosenbauer. “However, wild poliovirus type 2 has already been globally interrupted since 1999.” Inactivated polio vaccine, routinely used in the developed world, isn’t practical for use in polio-infected countries because it only offers individual immunity, Rosenbauer adds. However, given the availability of a new oral polio vaccine containing type 1 and 3 virus-strains only, “there is a suggestion to switch from trivalent OPV News CMAJ