The recent isolation of wild poliov

The recent isolation of wild poliovirus from Israeli sewage samples1 (environmental sampling) elucidates an important consideration for polio eradication. The Iocal population has humoral immunity against the poliovirus because of high rates of coverage with inactivated polio vaccine (IPV).2 But not everyone has mucosal (intestinal) immunity because of removal of oral polio vaccine (OPV) from routine immunisation since 2005, and absence of endemic wild poliovirus conferring natural immunity. People immunised with IPV are protected from disease but the poliovirus replicates in their intestines and is shed with stools for about 3 weeks after initial infection.3 As a result, with direct person-to-person transmission, wild poliovirus remains in circulation using transient carriers, which is happening at present in Israel.

This situation poses a number of potential threats to the eradication programme. Although Israel has high coverage with IPV, certain groups in the population refuse vaccinations4 and will not have immunity to the virus if exposed. Individuals having primary humoral immunodeficiencies will not only be at risk for the disease if infected, but could also be shedding the virus for years.5 The country can become a reservoir of wild poliovirus even though we might not see any cases of the paralytic disease. With movement of people within and across borders, the transient carriers can transport the virus to other areas and countries. IPV-only countries with high coverage can end up being in the same situation as Israel, and the virus can potentially circulate undetected because environmental testing is not done everywhere. Some countries, which have eradicated polio but have low immunisation coverage (eg, Yemen), can have resurgence of the virus and outbreaks of paralytic polio because no travel regulations for vaccination exist.

We cannot afford to ignore these threats. A mass vaccination with OPV in Israel is a potential solution but does not eliminate future risks. IPV-only countries need to consider the implications of this incident and perhaps modify their immunisation policies, and implement enhanced environmental sampling and regulations for international travellers to have OPV vaccination.