Association of body mass index with COVID-19 risk and vaccine effectiveness in children

Background Real-world COVID-19 risk and COVID-19 vaccine effectiveness (VE) data in children with obesity, as measured by body mass index (BMI), remain scarce. Our objectives were to study the association of BMI and incidence of COVID-19 and related hospitalization in children and evaluate the effectiveness of the mRNA COVID-19 vaccine (COMIRNATY, Pfizer-BioNTech) two-dose primary series and booster according to BMI. Methods We conducted a population-based cohort study of children aged 5–11 years in Singapore, from 1st September 2021 until 15th August 2023, covering the delta and omicron wave periods, using data extracted from national databases. Children were categorized according to BMI: underweight (BMI <5th centile), acceptable weight and overweight (BMI > 90th centile). The main outcomes were incidence of COVID-19 and related hospitalization. Multivariable adjusted analysis was conducted to measure association of outcomes and BMI category. Vaccine effectiveness (VE) was calculated using incidence rate ratios. Results A total of 255,477 children (48.7% male) were included in the study, with the following distribution: underweight, n = 17,612 (6.9%), acceptable weight, n = 211,819 (82.9%), and overweight, n = 26,046 (10.2%). Children who were overweight were more likely to be infected compared to children with acceptable weight during delta (OR 1.26, 95% confidence interval [CI] 1.18–1.34) and omicron periods (OR 1.20, 95% CI 1.17–1.23). Primary vaccination was effective against infection compared to unvaccinated (VE range 18–20%) and hospitalization (VE range 65–71%) in all 3 BMI categories. Booster dose improved slightly in children with overweight VE (VE 27%, 95% CI 10%–41%) but was not effective against hospitalization (VE 95% CI 0–58%). Compared to acceptable weight, cumulative COVID-19 incidence rate was higher in overweight regardless of vaccination throughout the study. Conclusions Children who were overweight had increased risk of COVID-19 throughout the delta and omicron periods. While primary series vaccination and booster dose were effective against infection in children who were overweight, despite vaccination, their incidence rate remained higher compared to vaccinated children with acceptable weight.