In February 2013, environmental surveillance in Israel (polio-free since 2003) documented introduction and subsequent sustained silent circulation until March 2014 of wild type 1 poliovirus into a population with very high vaccination coverage (>95% nationally, 90% at the epicenter - Bedouins in the Southern District) and 9 birth cohorts vaccinated exclusively with inactivate polio vaccine (IPV).
An emergency response team (ERT) supervised public health response throughout the event and provided evidence-based recommendations to prevent clinical poliomyelitis and to halt sustained transmission. Protection against disease was maximized through an IPV catch up campaign. The force of transmission remained high so the ERT, supported by WHO experts, recommended oral poliovaccine (OPV) for all children below age of 10 years in part due to the relative inferiority of IPV over OPV in preventing subsequent poliovirus infections and transmission. Safety considerations (reducing risk for serotype-2 VAPP and circulation of neurovirulent vaccine-derived polioviruses) and anticipation of the global ban of live type 2 vaccine strains by 2016 contributed to the choice of bivalent OPV (bOPV; type 1 and 3 vaccine strains) over trivalent OPV.
In conclusion, poliovirus can silently circulate in countries immunizing exclusively with IPV (poorer mucosal gut immunity) even when IPV coverage is very high. Vaccination response should incorporate IPV catch up for protection, followed by one or more round of bOPV or monovalent-OPV to halt transmission. Finally, adequate surveillance programs must be in place to detect events and monitor response.
Acknowledgment: Everyone who contributed towards this national public health emergency response.
