 The OPV is less expensive and easier to administer, and can spread immunity beyond the person vaccinated, creating contact immunity.It has been the predominant vaccine used.However, under conditions of long-term vaccine virus circulation in under-vaccinated populations, mutations can reactivate the virus to produce a polio-inducing strain, while the OPV can also, in rare circumstances, induce polio or persistent asymptomatic infection in vaccinated individuals, particularly those who are immunodeficient.