(345) Effectiveness of primary COVID-19 vaccination against severe infection in pediatrics: a cohort study using two linked data sources in Spain.

Background: Despite high SARS-CoV-2 infection incidences in the pediatric population in Spain, to date, around 8,900 cases led to hospitalization.

Objectives: We aimed to estimate the effectiveness of primary 2-dose COVID-19 vaccinations against hospitalization with COVID-19 in that population.

Methods: A cohort of individuals aged 5-14 years old (yo) between January 2021-February 2022 including Delta and Omicron variant periods in 2 data sources in Spain (eastern SIDIAP and multiregional BIFAP) who received a primary vaccination were matched 1:1 to unvaccinated controls based on the calendar date of 1st and 2nd doses, age, sex, region, and comorbidities (immunodeficiency, malignant tumor, transplant, severe renal disease and Down syndrome). Participants with a previous SARS-CoV-2 infection recorded were excluded to avoid modification of the vaccine effect. Hospitalised COVID-19 diagnoses (outcome) were identified in hospital records as one of the reasons (BIFAP) or as the main reason (in SIDIAP) for hospitalization with a positive SARS-CoV-2 test within 30 days. Incidence rate differences and vaccine effectiveness (VE; 95% CI; controlling confounders [chronic and acute comorbidities, influenza vaccination and other medication use and visits to primary care physician] by inverse probability weights) were calculated.

Results: The cohorts included 178,589 pairs (34,520 pairs of children aged 5-11 yo and 144,069 pairs of pre-adolescents aged 12-14 yo) in BIFAP during Delta and Omicron variant periods and 75,361 (109 and 75,252, respectively) in SIDIAP during Delta. Among vaccinated individuals, 21 hospitalizations with COVID-19 and 32 among controls were identified in BIFAP, whereas < 5 and 15 in SIDIAP, all pre-adolescents. We estimated that 0.5 (BIFAP) and 2.5 (SIDIAP) hospitalizations within the Delta period were prevented owing to vaccination and 0.8 with Omicron (BIFAP) per million pre-adolescent-days. The VE was 53% (95% CI: 18-74%; BIFAP, being only significant during the Omicron variant period) and 94% (52-99%; SIDIAP during Delta).

Conclusions: Full vaccination (2-doses; mainly with Pfizer) among pre-adolescents showed protection against hospitalizations with COVID-19 even though few cases were identified during Delta and the beginning of Omicron variants periods in Spain. The inclusion of hospitalizations ‘with’ instead of ‘for’ COVID-19 in BIFAP could have been underestimated the VE. Effectiveness reported was controlled by multiple factors, but differential detections (missing asymptomatic infections or positive self-tests) or characteristics among vaccinated and controls cannot be discarded that also depended on the calendar dates and virus variant.