Tolerability, safety, and immunogenicity of the novel oral polio vaccine type 2 in children aged 6 weeks to 59 months in an outbreak response campaign in The Gambia: an observational cohort study.

BACKGROUND: Novel oral polio vaccine type 2 (nOPV2) has been used to interrupt circulating vaccine-derived poliovirus type 2 outbreaks following its WHO emergency use listing. This study reports data on the safety and immunogenicity of nOPV2 over two rounds of a campaign in The Gambia. METHODS: This observational cohort study collected baseline symptoms (vomiting, diarrhoea, irritability, reduced feeding, and reduced activity) and axillary temperature from children aged 6 weeks to 59 months in The Gambia before a series of two rounds of a nOPV2 campaign that took place on Nov 20-26, 2021, and March 19-22, 2022. Serum and stool samples were collected from a subset of the participants. The same symptoms were re-assessed during the week following each dose of nOPV2. Stool samples were collected on days 7 and 28, and serum was collected on day 28 following each dose. Adverse events, including adverse events of special interest, were documented for 28 days after each campaign round. Serum neutralising antibodies were measured by microneutralisation assay, and stool poliovirus excretion was measured by real-time RT-PCR. FINDINGS: Of the 5635 children eligible for the study, 5504 (97·7%) received at least one dose of nOPV2. There was no increase in axillary temperature or in any of the baseline symptoms following either rounds of the campaigns. There were no adverse events of special interest and no other safety signals of concern. Poliovirus type 2 seroconversion rates were 70% (95% CI 62 to 78; 87 of 124 children) following one dose of nOPV2 and 91% (85 to 95; 113 of 124 children) following two doses. Poliovirus excretion on day 7 was lower after the second round (162 of 459 samples; 35·3%, 95% CI 31·1 to 39·8) than after the first round (292 of 658 samples; 44·4%, 40·6 to 48·2) of the campaign (difference -9·1%; 95% CI -14·8 to -3·3), showing the induction of mucosal immunity. INTERPRETATION: In a campaign in west Africa, nOPV2 was well tolerated and safe. High rates of seroconversion and evidence of mucosal immunity support the licensure and WHO prequalification of this vaccine. FUNDING: Bill & Melinda Gates Foundation.

Community-based survey to assess seroprevalence of poliovirus antibodies in far-north Cameroon in 2020.

Background: This study assessed seroprevalence of poliovirus antibodies in children from selected poliovirus high-risk areas of the Far North region of Cameroon which serves to monitor polio immunization program. Methods: This was a community-based cross-sectional seroprevalence survey involving collection of dried blood specimens (DBS) among children aged 12-59 months (n = 401). Multi-stage cluster sampling using GIS was applied to select the study sample. Collected DBS were analysed with microneutralization assays for poliovirus neutralizing antibody levels. Results: The overall seroprevalence of types 1, 2 and 3 neutralizing antibodies were 86.8 % (95 % confidence interval [CI]: 83.1-89.8), 74.6 % (95 % CI: 70.1-78.6) and 79.3 % (95 % CI: 75.1-83.0), respectively. Median titers (log2 scale) for type 1, 2 and 3 were 7.17 (6.5-7.5), 5.17 (4.83-5.5), and 6.17 (5.5-6.5), respectively. There was an increasing trend in median titers and seroprevalence with age, statistically significant between the youngest and oldest age groups (p < 0.001). Conclusion: Though there were several opportunities for vaccination through supplementary immunization activities (SIA) and routine immunization (RI), seroprevalence levels were low for all three serotypes, particularly for type 2. This highlights the need to strengthen RI and SIA quality coverage. Low population immunity makes Cameroon vulnerable to new importations and spread of polioviruses.