Estimates of SARS-CoV-2 vaccine effectiveness against COVID-19-associated hospitalisation in paediatric patients in Hong Kong during two successive SARS-CoV-2 epidemic waves dominated by the Omicron variant: A test-negative design study.

BACKGROUND

We assessed the effectiveness of SARS-CoV-2 vaccines against COVID-19-associated hospitalisation in paediatric patients in Hong Kong.

METHODS

We conducted a test-negative design study in hospitalised children 1-17 years old admitted with recent-onset (≤5 days) acute respiratory illness in two large public hospitals in Hong Kong during successive SARS-CoV-2 epidemic waves between April 2022 and February 2024 dominated by the Omicron variant. We defined having been vaccinated as receipt of ≥1 SARS-CoV-2 vaccine dose within six months prior to hospitalisation, and for a sensitivity analysis, within twelve months of hospitalisation. Children who had only received one dose of vaccine and had no prior infection were excluded. We estimated VE against laboratory-confirmed SARS-CoV-2 infection separately for 1-4-year-olds and 5-17-year-olds, by conditional logistic regression stratified by calendar time (week) and adjusted for potential confounders.

RESULTS

There were 6308 patients. The proportion of the vaccinated within six months of hospitalisation differed significantly between cases and controls in all strata of the assessed individual sociodemographic/health-related characteristics in the 5-17-year-olds, and in 1-4-year-olds who had a chronic medical condition. Overall, VE was moderate for 1-4-year-olds [41% (23-56%)] and for 5-17-year-olds [54% (10-66%)]. VE was higher in those who had received a mRNA vaccine: 87% (0-98%) and 82% (59-92%) compared with a lower VE for those who had received an inactivated vaccine: 39% (17-54%) and 30% (-19-58%), respectively for 1-4-year-olds and 5-17-year-olds although with a higher degree of imprecision in the VE for mRNA in 1-4-year-olds. In both age groups, VE was higher for those who had had no previous SARS-CoV-2 infection compared with for those who had had a previous infection.

CONCLUSIONS

SARS-CoV-2 vaccines, particularly mRNA vaccines, provided substantial protection against paediatric COVID-19-associated hospitalisations in Hong Kong during two successive SARS-CoV-2 epidemic waves dominated by the Omicron variant.