Genetic Analysis and Characterization of Wild Poliovirus Type 1 During Sustained Transmission in a Population With >95% Vaccine Coverage, Israel 2013

BACKGROUND: Israel has >95% polio vaccine coverage with the last 9 birth cohorts immunized exclusively with inactivated polio vaccine (IPV). Using acute flaccid paralysis and routine, monthly countrywide environmental surveillance, no wild poliovirus circulation was detected between 1989 and February 2013, after which wild type 1 polioviruses South Asia genotype (WPV1-SOAS) have persistently circulated in southern Israel and intermittently in other areas without any paralytic cases as determined by intensified surveillance of environmental and human samples. We aimed to characterize antigenic and neurovirulence properties of WPV1-SOAS silently circulating in a highly vaccinated population. METHODS: WPV1-SOAS capsid genes from environmental and stool surveillance isolates were sequenced, their neurovirulence was determined using transgenic mouse expressing the human poliovirus receptor (Tg21-PVR) mice, and their antigenicity was characterized by in vitro neutralization using human sera, epitope-specific monoclonal murine anti-oral poliovirus vaccine (OPV) antibodies, and sera from IPV-immunized rats and mice. RESULTS: WPV1 amino acid sequences in neutralizing epitopes varied from Sabin 1 and Mahoney, with little variation among WPV1 isolates. Neutralization by monoclonal antibodies against 3 of 4 OPV epitopes was lost. Three-fold lower geometric mean titers (Z = -4.018; P < .001, Wilcoxon signed-rank test) against WPV1 than against Mahoney in human serum correlated with 4- to 6-fold lower neutralization titers in serum from IPV-immunized rats and mice. WPV1-SOAS isolates were neurovirulent (50% intramuscular paralytic dose in Tg21-PVR mice: log10(7.0)). IPV-immunized mice were protected against WPV1-induced paralysis. CONCLUSIONS: Phenotypic and antigenic profile changes of WPV1-SOAS may have contributed to the intense silent transmission, whereas the reduced neurovirulence may have contributed to the absence of paralytic cases in the background of high population immunity.