Editorial commentary: emerging problems impeding the elimination of the last polioviruses: silent circulation of wild strains in a well-immunized population.

Despite high polio vaccine coverage, Israel recently experienced a silent surge in the transmission of wild poliovirus in the population. In this issue of Clinical Infectious Diseases, Shulman and colleagues report interesting data about the antigenic and pathogenic features of the poliovirus strain circulating in that country. Poliovirus, the causative agent of poliomyelitis, replicates mainly in the tonsils and in the small intestine. It is most often transmitted by the fecal–oral route and is extremely contagious. In most cases, replication of the virus in infected people remains asymptomatic or gives rise to only mild symptoms [1]. Paralytic poliomyelitis is relatively rare, affecting between 1 in 200 and 1 in 2000 infected people depending on the serotype of the virus (1, 2, and 3). Two vaccines effectively prevent poliomyelitis. The inactivated injectable polio vaccine (IPV) was developed by Jonas Salk through the chemical inactivation of neurovirulent strains of the 3 serotypes; it is very safe and induces a strong and protective general antibody response. The live attenuated oral polio vaccine (OPV) was developed by Albert Sabin and induces a long-term protective response; this response includes an effective intestinal immunity that restricts interhuman circulation and replication of the virus. However, a problematic aspect of OPV is a consequence of the genetic instability of the 3 attenuated strains (Sabin 1–3). Multiplication of OPV strains in vaccinees allows their genetic drift and loss of attenuation. Consequently, neurovirulent vaccine-derived polioviruses (VDPVs) can be excreted for years by chronically infected immunodeficient individuals (iVDPVs). In addition, pathogenic circulating vaccine-derived polioviruses (cVDPVs) can emerge following the interhuman circulation of OPV strains in underimmunized populations [2]. Nevertheless, as OPV can be distributed by nonmedical personnel, is cheap, and, above all, limits the interhuman circulation of the virus, it has been used as the main tool of the vaccination campaigns in developing countries, part of the global poliomyelitis eradication initiative launched by the World Health Organization (WHO) in 1988. Surveillance of the disease has been mainly based on virological investigations of stool samples from patients with acute flaccid paralysis (AFP). Few infected people develop the paralytic disease, so it is essential to detect, accurately and rapidly, all AFP cases so as to facilitate programs of vaccination with OPV as quickly as possible and thereby stop outbreaks by interrupting poliovirus circulation. This AFP surveillance is implemented by a global network of >150 laboratories, all using standardized methodologies and reagents. Vaccination campaigns and surveillance have been largely effective: the incidence of poliomyelitis worldwide has decreased by >99% since 1988. The wild type 2 strains have been eradicated, and only 3 countries in the world (Nigeria, Pakistan, and Afghanistan) are still endemic for poliomyelitis due to local wild viruses. In addition, the recent eradication of poliovirus from India and the probable disappearance of wild type 3 viruses from the planet allow us to be reasonably optimistic about the eradication of the last wild type 1 polioviruses in the near future [3]. Nevertheless, over the last 15 years, the program has been facing serious issues: first, it has been difficult to improve polio vaccine coverage in the last endemic countries; and second, maintaining satisfactory Received 1 December 2014; accepted 3 December 2014; electronically published 30 December 2014. Correspondence: Francis Delpeyroux, PhD, Institut Pasteur, Biologie des Virus Entériques, 75724 Paris cedex 15, France ([email protected]). Clinical Infectious Diseases 2015;60(7):1065–7 © The Author 2014. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. [email protected]. DOI: 10.1093/cid/ciu1139