A randomized phase 4 study of immunogenicity and safety following monovalent oral type 2 Sabin polio vaccine challenge in IPV-vaccinated children in Lithuania.

BACKGROUND Understanding immunogenicity and safety of monovalent type-2 oral polio vaccine (mOPV2) in inactivated polio vaccine (IPV)-immunized children is of major importance to inform global policy to control circulating vaccine-derived poliovirus (cVDPV) outbreaks. METHODS In this open-label, phase 4 study (NCT02582255) in 100 IPV-vaccinated Lithuanian 1-5-year-olds we measured humoral and intestinal type-2 polio neutralizing antibodies before and 28 days after one or two mOPV2 doses given 28 days apart, and stool viral shedding after each dose. Parents recorded solicited adverse events (AE) for 7 days after each dose and unsolicited AEs for 6 weeks postvaccination. RESULTS After one mOPV2 challenge the type-2 seroprotection rate increased from 98% to 100%. Approximately 28 days after mOPV2 challenge 34 of 68 (50%, 95% CI: 38-62) children were shedding virus; 9 of 37 (24%, 12-41) were shedding 28 days after a second challenge. Before challenge type-2 intestinal immunity was undetectable in IPV-primed children, but 28 of 87 (32%) had intestinal neutralizing titers ≥ 32 after one mOPV2 dose. No vaccine-related serious or severe AEs were reported. CONCLUSIONS High viral excretion following mOPV2 among exclusively IPV-vaccinated children was substantially lower following a subsequent dose, indicating induction of intestinal immunity against type-2 poliovirus.