Safety and effectiveness of monovalent COVID-19 mRNA vaccination and risk factors for hospitalisation caused by the omicron variant in 0.8 million adolescents: A nationwide cohort study in Sweden

• Published in: Nature methods Vol. 20; no. 2; p. e1004127
• Main Authors: Nordström, Peter, Ballin, Marcel, Nordström, Anna
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 21.02.2023; PLOS; Public Library of Science (PLoS)
• Subjects: Adolescent; Adolescents; Bacterial infections; Biology and Life Sciences; Cohort analysis; Cohort Studies; Coronaviruses; COVID-19; COVID-19 - epidemiology; COVID-19 - prevention & control; COVID-19 vaccines; Developmental disabilities; Drug therapy; Hospital care; Hospitalization; Humans; Immunization; Infections; Influenza; Medicine and Health Sciences; Messenger RNA; mRNA; mRNA vaccines; Observational studies; Paralysis; People and places; Public health; Regression analysis; Risk Factors; Risk groups; RNA, Messenger; Safety; Safety and security measures; SARS-CoV-2; Severe acute respiratory syndrome coronavirus 2; Statistics; Sweden - epidemiology; Teenagers; Tonsillitis; Vaccination; Vaccine efficacy; Youth; Sweden; United States > US
• ISSN: 1549-1676; 1549-1277; 1548-7091; 1548-7105; 1549-1676; 1548-7105
• DOI: 10.1371/journal.pmed.1004127

Real-world evidence on the safety and effectiveness of Coronavirus Disease 2019 (COVID-19) vaccination against severe disease caused by the omicron variant among adolescents is sparse. In addition, evidence on risk factors for severe COVID-19 disease, and whether vaccination is similarly effective in such risk groups, is unclear. The aim of the present study was therefore to examine the safety and effectiveness of monovalent COVID-19 mRNA vaccination against COVID-19 hospitalisation, and risk factors for COVID-19 hospitalisation in adolescents. A cohort study was conducted using Swedish nationwide registers. The safety analysis included all individuals in Sweden born between 2003 and 2009 (aged 11.3 to 19.2 years) given at least 1 dose of monovalent mRNA vaccine (N = 645,355), and never vaccinated controls (N = 186,918). The outcomes included all-cause hospitalisation and 30 selected diagnoses until 5 June 2022. The vaccine effectiveness (VE) against COVID-19 hospitalisation, and risk factors for hospitalisation, were evaluated in adolescents given 2 doses of monovalent mRNA vaccine (N = 501,945), as compared to never vaccinated controls (N = 157,979), for up to 5 months follow-up during an omicron predominant period (1 January 2022 to 5 June 2022). Analyses were adjusted for age, sex, baseline date, and whether the individual was born in Sweden. The safety analysis showed that vaccination was associated with 16% lower (95% confidence interval (CI) [12, 19], p < 0.001) risk of all-cause hospitalisation, and with marginal differences between the groups regarding the 30 selected diagnoses. In the VE analysis, there were 21 cases (0.004%) of COVID-19 hospitalisation among 2-dose recipients and 26 cases (0.016%) among controls, resulting in a VE of 76% (95% CI [57, 87], p < 0.001). Predominant risk factors for COVID-19 hospitalisation included previous infections (bacterial infection, tonsillitis, and pneumonia) (odds ratio [OR]: 14.3, 95% CI [7.7, 26.6], p < 0.001), and cerebral palsy/development disorders (OR: 12.7, 95% CI [6.8, 23.8], p < 0.001), with similar estimates of VE in these subgroups as in the total cohort. The number needed to vaccinate with 2 doses to prevent 1 case of COVID-19 hospitalisation was 8,147 in the total cohort and 1,007 in those with previous infections or developmental disorders. None of the individuals hospitalised due to COVID-19 died within 30 days. Limitations of this study include the observational design and the possibility of unmeasured confounding. In this nationwide study of Swedish adolescents, monovalent COVID-19 mRNA vaccination was not associated with an increased risk of any serious adverse events resulting in hospitalisation. Vaccination with 2 doses was associated with a lower risk of COVID-19 hospitalisation during an omicron predominant period, also among those with certain predisposing conditions who should be prioritised for vaccination. However, COVID-19 hospitalisation in the general population of adolescents was extremely rare, and additional doses in this population may not be warranted at this stage.