POPULATION AND AGE-SPECIFIC TIMING OF COVID-19 VACCINATION IN HONG KONG: A TIME-TO-EVENT ANALYSIS

Abstract

Intro: Through mass vaccination, rapid attainment of population immunity can be achieved to create community defence against impending outbreaks. This study characterised the timing of COVID-19 vaccination coverage at population level and evaluated its association with age in Hong Kong. Methods: In an ongoing population-based cohort, participant's demographics, histories of COVID-19 vaccination and diagnosis were collected as of June 2022. For both the second and third/booster dose, the average time to achieve 50% coverage in the cohort was estimated using Kaplan-Meier method. Multivariable cox regression model was applied to determine the age effects on vaccination timing. Findings: Among 4668 participants (42% male, median age: 46 years [IQR 34-57]), 93% and 68% have received the second and third/booster dose against COVID-19, respectively. The second-dose 50% coverage was achieved at 164 days after vaccination roll-out. Uptake decelerated when 75% participants were vaccinated, after which it took an additional 163 days to augment the coverage to 90%. Second-dose uptake did not differ significantly by age over time, but a notable gap was observed between those aged 65+ (34%) and 18-29 years (15%) in the first 100 days. Between November 2021 and March 2022 when the 180-day dose-interval was effective, half of the participants received their third/booster dose in less than 54 days after they became eligible. Stratified by age, 50% subjects aged 18-29, 30-39, 40-49, 50-64 and 65+ years received the vaccine within 82, 73, 67, 45 to 26 days, respectively. A consistently higher uptake of third/booster dose was identified in those aged 40-49 (adjusted hazard ratio[aHR] 1.21), 50-64 (aHR 1.19) and 65+ (aHR 1.19) compared to 18-29 years adjusted for demographics. Conclusion: This study highlighted the need of tracking the age pattern of immunity against SARS-CoV-2 and tailoring strategies to expedite vaccination in younger adults. The heterogeneous pattern of vaccination timing also informed future vaccination campaigns.