Post-vaccination infection rates and modification of COVID-19 symptoms in vaccinated UK school-aged children and adolescents: A prospective longitudinal cohort study

Abstract

Background: We aimed to explore the effectiveness of one-dose BNT162b2 vaccination upon SARS-CoV-2 infection, its effect on COVID-19 presentation, and post-vaccination symptoms in children and adolescents (CA) in the UK during periods of Delta and Omicron variant predominance.

Methods: In this prospective longitudinal cohort study, we analysed data from 115,775 CA aged 12-17 years, proxy-reported through the Covid Symptom Study (CSS) smartphone application. We calculated post-vaccination infection risk after one dose of BNT162b2, and described the illness profile of CA with post-vaccination SARS-CoV-2 infection, compared to unvaccinated CA, and post-vaccination side-effects.

Findings: Between August 5, 2021 and February 14, 2022, 25,971 UK CA aged 12-17 years received one dose of BNT162b2 vaccine. The probability of testing positive for infection diverged soon after vaccination, and was lower in CA with prior SARS-CoV-2 infection. Vaccination reduced proxy-reported infection risk (-80·4% (95% CI -0·82 -0·78) and -53·7% (95% CI -0·62 -0·43) at 14-30 days with Delta and Omicron variants respectively, and -61·5% (95% CI -0·74 -0·44) and -63·7% (95% CI -0·68 -0.59) after 61-90 days). Vaccinated CA who contracted SARS-CoV-2 during the Delta period had milder disease than unvaccinated CA; during the Omicron period this was only evident in children aged 12-15 years. Overall disease profile was similar in both vaccinated and unvaccinated CA. Post-vaccination local side-effects were common, systemic side-effects were uncommon, and both resolved within few days (3 days in most cases).

Interpretation: One dose of BNT162b2 vaccine reduced risk of SARS-CoV-2 infection for at least 90 days in CA aged 12-17 years. Vaccine protection varied for SARS-CoV-2 variant type (lower for Omicron than Delta variant), and was enhanced by pre-vaccination SARS-CoV-2 infection. Severity of COVID-19 presentation after vaccination was generally milder, although unvaccinated CA also had generally mild disease. Overall, vaccination was well-tolerated.

Funding: UK Government Department of Health and Social Care, Chronic Disease Research Foundation, The Wellcome Trust, UK Engineering and Physical Sciences Research Council, UK Research and Innovation London Medical Imaging & Artificial Intelligence Centre for Value Based Healthcare, UK National Institute for Health Research, UK Medical Research Council, British Heart Foundation and Alzheimer's Society, and ZOE Limited.

Keywords: BNT162b2 vaccine effectiveness; BNT162b2, Comirnaty SARS-CoV-2 vaccine (BioNTech, Pfizer); CA, Children and adolescents; COVID-19 vaccination; KCL, King's College London; LFAT, Lateral flow antigen test; OR, Odds Ratio; PCR, Polymerase chain reaction; Paediatrics; SARS-CoV-2 vaccination; SARS-CoV-2 vaccination in children; SARS-CoV-2, Severe acute respiratory syndrome‐related coronavirus 2; UK, United Kingdom of Great Britain and Northern Ireland.

Figure 1

Flowchart of participants according to inclusion and exclusion criteria for this study. Entire cohort is given first, before subdivision into vaccinated and unvaccinated CA. Note here the two age groups had differing time periods for data consideration (adolescents aged 16–17 years: able to receive vaccination after August 4, 2021; children aged 12-15 years: able to receive vaccination after September 18, 2021). Further subdivision provides numbers of individuals for periods of Delta and Omicron variant predominance in UK. Considered time frames for Delta and Omicron variant predominance were not contiguous; thus, numbers of CA within each time frame do not sum to numbers across the entire study period. Not valid result = test result proxy-reported as “failed test” or “still waiting”. Irregular logging = proxy-reporting with intervals of more than 7 days between proxy-reports during illness. PCR – polymerase chain reaction. LFAT – lateral flow antigen test.

Figure 2

Survival analysis: Kaplan-Meier plots showing probability of reporting no positive result when tested for SARS-CoV-2 infection over time, for vaccinated and unvaccinated CA without prior SARS-CoV-2 infection, during periods of Delta (left panel) and Omicron (right panel) variant predominance.

Figure 3

Survival analysis: Kaplan-Meier plots showing the probability of reporting no positive result at tests for SARS-CoV-2 infection over time for vaccinated CA with or without SARS-CoV-2 infection prior to vaccination, during periods of Delta (left panel) and Omicron (right panel) variant predominance.

Figure 4

Illness profile in vaccinated and unvaccinated CA testing positive for SARS-CoV-2 during periods of Delta (top panels) and Omicron (bottom panels) variant predominance, in previously SARS-CoV-2-naÿve children (left panels) and adolescents (right panels). Asymptomatic CA are included in this figure.

Figure 5

Odds ratios for symptom prevalence in vaccinated vs. unvaccinated CA first testing positive for SARS-CoV-2 during periods of Delta (left) and Omicron (right) variant predominance. Vaccinated CA had no reported SARS-CoV-2 infection prior to vaccination. Asymptomatic CA testing positive for SARS-CoV-2 are included in the computation. Red colour encodes significant p-values after false discovery rate correction (alpha =0.05).

Figure 6

Proportion of CA aged 12 to 17 years old, separated by age group, reporting vaccine side-effects after one dose of BNT162b2. Left panels: local symptoms at the arm of injection within 7 days; right panels: systemic symptoms presenting within the same 7 days. Data for children aged 12-15 years are shown in upper panels and for adolescents aged 16-17 years in lower panels.