What role for inactivated poliovirus vaccine in the eradication endgame?

The 15-decade drive to control and eradicate polio through immunization [1] has benefited from the availability of 2 excellent vaccines, the inactivated poliovirus vaccine (IPV) of Salk and colleagues, licensed in 1955, and the live attenuated oral poliovirus vaccine (OPV) of Sabin, licensed in 1961. The 2 vaccines have complementary advantages, and both have played—and continue to play—important roles in polio control. IPV has been used primarily in developed countries with good sanitation and temperate climates, whereas OPV has been the primary weapon in the World Health Organization (WHO) initiative to eradicate polio from the developing world [2, 3]. The introduction of IPV in the United States and other high-income developed countries was followed by a sharp decline in polio incidence, but IPV was gradually replaced by OPV in the early 1960s, with only The Netherlands, Nordic countries, and some Canadian provinces continuing exclusive use of IPV. OPV offers the promise of global polio eradication because of its efficient induction of intestinal immunity, ease of administration, suitability for mass immunization campaigns , and lower cost. Great strides have been made toward the goal of global polio eradication, with the apparent eradication of wild poliovirus type 2 in 1999 [4]; the highly localized en-demic circulation of wild poliovirus type 3 to pockets in southern Afghanistan, northern India, and northern Nigeria; and the dramatic progress in the control of wild poliovirus type 1 in Egypt and southern Asia (updates are posted on the WHO Web site, http://www.polioeradication .org/). The incidence of polio has declined from an estimated 350,000 cases in 1988 to 1948 cases in 2005. However, wild po-liovirus type 1 from reservoirs in northern Nigeria, where polio immunization had been interrupted in 2003 and 2004, spread to 18 previously polio-free countries in 2003–2005, from Guinea in the west, to Yemen and Somalia in the Horn of Africa, to Indonesia at the southeastern edge of Asia, resulting in a steep increase in the number of cases last year [5]. Nationwide mass immunization campaigns with OPV have rolled back polio in most of these countries, but outbreaks continue in In-donesia, Somalia, and Ethiopia, while northern Nigeria remains an active reservoir for poliovirus types 1 and 3. These recent events underscore the urgency of completing the task of eradicating all wild polioviruses. The success of the WHO initiative has fundamentally altered poliovirus ecology, to the point at which the only source of immunity to …