Characterization of mutations in the VP sub(1) region of Sabin strain type 1 polioviruses isolated from vaccine-associated paralytic poliomyelitis cases in Iran

Background: The live-attenuated oral polio vaccine used to interrupt poliovirus transmission is genetically unstable. Reversion of some attenuating mutations, which normally occurs during vaccine strain replication in some recipients, and can rarely cause vaccine-associated paralytic poliomyelitis (VAPP). The poliovirus eradication program designed by the World Health Organization (WHO) includes immunization with OPV in addition to careful surveillance of all acute-flaccid paralysis (AFP) cases. Objectives: In Iran we last isolated imported wild poliovirus in 2000 and the immunization coverage was 100% in 2002. During 2001, there were three AFP cases with residual paralysis from which Sabin-like type 1 polioviruses were isolated in our national polio laboratory. Study design: The complete VP sub(1) region of the three isolates was sequenced and amino acid substitutions associated with these neurovirulent isolates were recorded. Results: These isolates had either 4, 2 or 1 nucleotide substitution(s) in the VP sub(1) region, corresponding to amino acid change in the VP sub(1) of isolate 1 of either (H approximately equal to 149Y), (T approximately equal to 106A) or (I approximately equal to 90L), respectively. Conclusions: Surveillance of the VAPP cases in countries where endemic transmission has recently ceased increases our understanding of the important neurovirulent mutations in vaccine-strain isolates and assists in planning the next step in the eradication program in these countries.