Case Report: Prolonged nOPV2 shedding in an immunocompetent child from Tunisia border with Algeria, North African region

Following the detection of vaccine-derived poliovirus type 2 (cVDPV2) in Algeria in 2022, enhanced surveillance activities along the Tunisian border identified novel Oral Poliovirus type 2 Vaccine (nOPV2) excretion in an apparently healthy nomadic child. Immunological investigations showed no evidence of major humoral, cellular, or phagocytic immune deficiency. To investigate the kinetics of viral shedding and characterize the complete genome of the isolated strain in this case report, longitudinal follow-up stool samples were collected and analyzed using cell culture inoculation, real-time RT-PCRs, and whole-genome sequencing. Virus was detected over a 76 to 126-days observation period. Assuming vaccination occurred in December 2022, the total duration of shedding may have extended up to approximately 138 to 188 days (4 to 7 months) post-vaccination. Despite prolonged replication, the viruses showed limited within-host evolution and retained the major engineered attenuation features of nOPV2, with no evidence of domain V reversion, cre replacement, or recombination, and only limited VP1 divergence which allows its classification as nOPV2 category 8 strains. Our findings underscore the need to further investigate the potential for prolonged excretion and molecular evolution of nOPV2-like strains in immunocompetent individuals. Furthermore, targeted surveillance of high-risk and mobile populations, including migrants and nomadic communities, is essential to strengthen epidemiological security and support global poliovirus eradication initiatives.