• Tim K Tsang, Sheena G Sullivan, Yu Meng, Francisco Tsz Tsun Lai, Min Fan, Xiaotong Huang, Yun Lin, Liping Peng, Chengyao Zhang, Bingyi Yang, Kylie E C Ainslie, and Benjamin J Cowling.
• WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing, Faculty of Medicine, The University of Hong Kong, 7 Sassoon Road, Pokfulam, Hong Kong Special Administrative Region, China. timtsang@connect.hku.hk.
• Bmc Med. 2024 Sep 12; 22 (1): 384384.

BackgroundExtending the dosing interval of a primary series of mRNA COVID-19 vaccination has been employed to reduce myocarditis risk in adolescents, but previous evaluation of impact on vaccine effectiveness (VE) is limited to risk after second dose.MethodsWe quantified the impact of the dosing interval based on case notifications and vaccination uptake in Hong Kong from January to April 2022, based on calendar-time proportional hazards models and matching approaches.ResultsWe estimated that the hazard ratio (HR) and odds ratio (OR) of infections after the second dose for extended (28 days or more) versus regular (21-27 days) dosing intervals ranged from 0.86 to 0.99 from calendar-time proportional hazards models, and from 0.85 to 0.87 from matching approaches, respectively. Adolescents in the extended dosing groups (including those who did not receive a second dose in the study period) had a higher hazard of infection than those with a regular dosing interval during the intra-dose period (HR 1.66; 95% CI 1.07, 2.59; p = 0.02) after the first dose.ConclusionsImplementing an extended dosing interval should consider multiple factors including the degree of myocarditis risk, the degree of protection afforded by each dose, and the extra protection achievable using an extended dosing interval.© 2024. The Author(s).

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