WHO Upgrades Vaccine-Derived Poliovirus Data

Vaccine-associated poliovirus infection is on a downswing, but the World Health Organization's (WHO's) Endgame and Strategic Plan is still vital to prevent transmission from individuals with immunodeficiency-related cases who excrete live virus for years, according to an update on surveillance data published in the June 19 issue of the Morbidity and Mortality Weekly Report.

Vaccine-Derived Poliovirus Data

Ousmane M. Diop, PhD, from the Department of Immunization, Vaccines, and Biologicals, WHO, Geneva, Switzerland, and colleagues analyzed data on poliovirus infection from January 2014 through March 2015. Most cases of poliovirus infection derive from the oral polio vaccine (OPV), which consists of three serotypes (1, 2, and 3) of attenuated virus.

Circulating vaccine-derived polioviruses (cVDPVs) are genetically distinct from their wild cousins, but can cause outbreaks, particularly in areas where immunization is incomplete. In addition, there are people with primary (inherited) immunodeficiency-associated vaccine-derived polio infection (iVDPVs), who excrete live virus — in one case for more than 2 decades. The WHO's Global Polio Eradication Initiative maintains a registry of immunodeficiency-related cases, which are rare.

Serotype 2 accounts for more than 97% of cVDPVs detected since 2006 and 65% of iVDPVs detected since 1962 and dampens immunity to serotypes 1 and 3. Therefore, the WHO's plan will remove it from OPV.

From January 2014 to March 2015, cVDPV transmission has diminished, the new study reports. Outbreaks in Afghanistan, Chad, Somalia, and Yemen have apparently stopped, and the large outbreak in Nigeria has nearly ended. The outbreak in Pakistan from June 2014 waned, but a new, small emergence appeared in early 2015. Madagascar and South Sudan had limited outbreaks.

Nine individuals in seven countries were identified who excreted iVDPVs. Comprehensive inactivated polio vaccine (IPV) can prevent these rare persistent cases from seeding an outbreak. The report includes details of the cases.

The WHO's goal is to switch by April 2016 from trivalent to bivalent OPV after at least one dose of IPV, given as part of the routine immunization schedule, globally. This approach will maintain protection against serotype 2. Without IPV, continued use of OPV would enable immunodeficiency-related infections to arise.

In addition to the vaccine protocols, WHO's strategy includes surveillance and a response plan for cases of acute flaccid paralysis and development of antiviral drugs to treat persistent immunodeficiency-associated cases.

A limitation of the study is the difficulty of distinguishing closely related polioviruses from each other and from other enteroviruses found in sewage samples, and extrapolating those findings to specific cases.

The researchers conclude, "Replacement of [trivalent OPV] with [bivalent OPV] will greatly reduce the risk for cVDPV2 outbreaks, and global cessation of OPV use will ultimately prevent all cVDPV outbreaks and all new iVDPV infections." They caution that complete IPV coverage will remain necessary, despite the increasing rarity of infection, to prevent new cases from people with immunodeficiency who may excrete poliovirus for years after the final OPV dose.

The researchers have disclosed no relevant financial relationships.