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BackgroundAlthough paediatric clinical presentations of COVID-19 are usually less severe than in adults, serious illness and death have occurred. Many countries started the vaccination rollout of children in 2021; still, information about effectiveness in the real-world setting is scarce. The aim of our study was to evaluate vaccine effectiveness (VE) against COVID-19-associated-hospitalisations in the 3-17-year population during the Omicron outbreak.MethodsWe conducted a retrospective cohort study including individuals aged 3-17 registered in the online vaccination system of the Buenos Aires Province, Argentina. mRNA-1273 and BNT162b2 were administered to 12-17-year subjects; and BBIBP-CorV to 3-11- year subjects. Vaccinated group had received a two-dose scheme by 12/1/2021. Unvaccinated group did not receive any COVID-19 vaccine between 12/14/2021-3/9/2022, which was the entire monitoring period. Vaccine effectiveness (VE) against COVID-19-associated hospitalisations was calculated as (1-OR) x100.FindingsBy 12/1/2021, 1,536,435 individuals aged 3-17 who had received zero or two doses of SARS-CoV-2 vaccines were included in this study. Of the latter, 1,440,389 were vaccinated and 96,046 not vaccinated. VE were 78·0% [68·7-84·2], 76·4%[62·9-84·5] and 80·0%[64·3-88·0] for the entire cohort, 3-11 subgroup and 12-17 subgroup, respectively. VE for the entire population was 82·7% during the period of Delta and Omicron overlapping circulation and decreased to 67·7% when Omicron was the only variant present.InterpretationThis report provides evidence of high vaccine protection against associated-hospitalisations in the paediatric population during the Omicron outbreak but suggests a decrease of protection when Omicron became predominant. Application of a booster dose in children aged 3-11 warrants further consideration.Research in contextEvidence before this studyThere is limited evidence on the effectiveness of vaccines in the pediatric population, particularly in children aged 3-11 years after the SARS-CoV-2 B.1.1.529 (Omicron) variant’s emergence.We searched preprint and peer-reviewed published articles in PubMed, medRxiv, and SSRN for observational studies, with no language restrictions, using the term “COVID-19 OR SARS-CoV-2” AND “vaccine effectiveness” OR “vaccine impact” AND “children” OR “pediatric” AND “Omicron” published between December 1, 2021, and April 1, 2022. We found 4 studies that included subjects in the 3-17-year population who received a two-dose-scheme of any of the available vaccines-according to each country’s authorisation. Three studies were from the US; two were test-negative-case-control studies and one was a retrospective non-peer-reviewed cohort study. The reported vaccine effectiveness (VE) for 2-doses of BNT162b2-mRNA (Pfizer-BioNTech) in preventing hospitalisations during Omicron predominance was 48-78%; and it was 40-92% for 5-11 and 12-17-year subgroups, respectively. The fourth was a cohort study still in preprint form conducted in Chile and utilized an inactivated vaccine, CoronaVac (SinoVac), widely used in Latin-America. VE for two doses of CoronaVac in the 3-5-year subgroup against hospitalisations was 64% and 69% against ICU admissions.Added value of this studyUp to date, there are no published studies about the effectiveness of the BBIBP-CorV vaccine against hospitalisation in the pediatric population. Additionally, there are no real-world studies from low and middle-income countries about VE in the 12-17 aged population during the Omicron outbreak.This study shows that VE after 14 days or more from two-dose-scheme was 78·0% [68·7-84·2], 76·4% [62·9-84·5] and 80·0% [64·3-88·0] for the 3-17-year entire group, and for 3-11-year (BBIBP-CorV) and 12-17-year (mRNA vaccines) subgroups, respectively. VE for the 3-17-year entire group was 82·7% during the period of Delta and Omicron overlapping circulation and decreased to 67·7% when Omicron was the only variant present. These effects were consistent across all subgroups.Implications of all the available evidenceOur results provide evidence of high vaccine protection against COVID-19 associated-hospitalisations in the pediatric population during the Omicron outbreak, but suggest a decrease of protection when Omicron became predominant. Application of a booster dose in children aged 3-11 warrants further consideration.

Although paediatric clinical presentations of COVID-19 are usually less severe than in adults, serious illness and death have occurred. Many countries started the vaccination rollout of children in 2021; still, information about effectiveness in the real-world setting is scarce. The aim of our study was to evaluate vaccine effectiveness (VE) against COVID-19-associated-hospitalisations in the 3-17-year population during the Omicron outbreak.We conducted a retrospective cohort study including individuals aged 3-17 registered in the online vaccination system of the Buenos Aires Province, Argentina. mRNA-1273 and BNT162b2 were administered to 12-17-year subjects; and BBIBP-CorV to 3-11-year subjects. Vaccinated group had received a two-dose scheme by 12/1/2021. Unvaccinated group did not receive any COVID-19 vaccine between 12/14/2021 and 3/9/2022, which was the entire monitoring period. Vaccine effectiveness (VE) against COVID-19-associated hospitalisations was calculated as (1-OR)x100.By 12/1/2021, 1,536,435 individuals aged 3-17 who had received zero or two doses of SARS-CoV-2 vaccines were included in this study. Of the latter, 1,440,389 were vaccinated and 96,046 not vaccinated. VE were 78.0%[68.7-84.2], 76.4%[62.9-84.5] and 80.0%[64.3-88.0] for the entire cohort, 3-11-year (BBIBP-CorV) subgroup and 12-17 (mRNA vaccines) subgroup, respectively. VE for the entire population was 82.7% during the period of Delta and Omicron overlapping circulation and decreased to 67.7% when Omicron was the only variant present.This report provides evidence of high vaccine protection against associated hospitalisations in the paediatric population during the Omicron outbreak but suggests a decrease of protection when Omicron became predominant. Application of a booster dose in children aged 3-11-year warrants further consideration.None.

An extract of Calea lantanoides which inhibits larval development of Strongyloides stercoralis and three species of hookworms gave the heliangolide 15-deoxybudlein A.