A concern is the presence of circulating vaccine-derived polioviruses. The oral polio vaccine is not perfect: while the genetic characteristics are carefully balanced to maximize efficacy and minimize virulence, it is possible for the polio virus in the oral vaccine to mutate. As a result, persons given the oral polio vaccine can acquire acute or chronic infections; or can transmit (circulate) mutated virus to other people. Circulating vaccine-derived poliovirus cases have exceeded wild-type cases, making it desirable to discontinue use of the oral polio vaccine as soon as safely possible.