Background: Extended intervals between the first and second doses of mRNA Covid-19 vaccines may reduce the risk of myocarditis in children and adolescents. However, research exploring the impact of this extension on vaccine effectiveness remains scant.
Objectives: To investigate whether vaccine effectiveness would be affected after extending the dosing interval between the first and second of BNT162b2 among children and adolescents.
Methods: Using territory-wide electronic health records in Hong Kong, we conducted a nested case-control study. Adolescents aged from 3 to 17 years old who received two-dose BNT162b2 were included. Study outcomes include the first recorded Covid-19 infection and Covid-19-related hospitalization. By incidence density sampling, up to four controls were randomly matched with each case by age and sex. Multivariable conditional logistic regression was conducted to estimate adjusted odds ratios (aOR).This study was approved by the Central Institutional Review Board of the Hospital Authority of Hong Kong (CIRB-2021-005-4) and the Department of Health Ethics Committee (LM171/2021).
Results: From January to August 2022, 5396 Covid-19 cases and 202 Covid-19 related hospitalizations were identified and matched with 21577 and 808 controls, respectively. Compared to regular dosing intervals, there is a 29.2%-reduced risk of Covid-19 infection for the vaccine recipients with extended intervals [aOR 0.718, 95% Confidence Interval (CI): 0.619, 0.833]. Differences in hospitalization between regular and extended dosing intervals were not significant (aOR 0.743, 95%CI: 0.338, 1.636).
Conclusions: Greater protection against Covid-19 infection was observed after extending the dosing interval between the priming first two doses of BNT162b2, compared with a regular interval. Longer dosing intervals should be considered for children and adolescents.
