在剛過去的聖誕長假期及除夕，新冠病毒變異病毒株Omicron在香港社區引起一系列的傳播鏈。目前已有數據顯示，Omicron的傳播能力較原毒株或其他變異株高，而疫苗對Omicron的中和能力亦較對原毒株低。Omicron疫情一觸即發，情況令人憂慮。有見及此，香港中文大學（中大）醫學院賽馬會公共衞生及基層醫療學院、香港城巿大學（城大）賽馬會動物醫學及生命科學院領導的研究團隊，利用數學模型評估本港在大部分巿民已接種疫苗的情況下，Omicron帶來的潛在風險。
假設 (i) 已有約65%人口接種至少一劑疫苗、(ii) 接種兩劑疫苗的有效保護感染比率為50%（科興）及70%（復必泰）、(iii) Omicron基本傳染數 (R0) 為 8（當人口沒有抗體、沒有使用疫苗或沒有實施社交距離措施的情況下）、(iv) 抗體隨時間下降、(v) 追蹤檢測效能與第四波疫情一樣，及 (vi) Omicron重症率為Delta的一半，研究組就以下兩個情況為香港疫情作估算：
若社交接觸程度一直維持在過去兩星期（包括聖誕及元旦假期）的水平，由Omicron引起的第五波疫情將有超過300萬人受感染，及超過17萬人得重症；
由一月七日起生效的收緊社交距離措施，以致社交接觸程度較過去兩星期的水平
2.1. 減少25%（即每天平均接觸的人數降至14），第五波疫情會有大約25萬人受感染，及大約1.5萬人得重症；
2.2. 減少40%（即每天平均接觸的人數降至11），第五波疫情會有大約1.25萬人受感染，及大約700人得重症；
2.3. 減少到疫情以來的最低點（即2020年7至8月的第三波期間），即減少55%（即每天平均接觸的人數降至8），第五波疫情會有大約1,100人受感染，其中大約50人得重症。
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During the past Christmas and New Year holidays, Omicron, the variant of SARS-CoV-2, has been establishing a series of transmission chains in the Hong Kong community. Current data reveals that Omicron has a higher transmissibility than the original strain or other variants, and the neutralising ability of the vaccine against Omicron is also lower than that against the original strain. The Omicron epidemic is looming, and the situation is worrying. In the light of this, the research team led by JC School of Public Health and Primary Care, at The Chinese University of Hong Kong’s (CUHK) Faculty of Medicine (CU Medicine), and the Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong Kong are using a mathematical modelling approach to assess the possible risks of Omicron outbreaks in a majority vaccinated population in Hong Kong.
This study employs an age stratified mathematical model using social mobility levels as proxy estimate of social contact, taking into account: (i) the age-specific susceptibility to COVID-19 infection, (ii) the effectiveness of the two COVID-19 vaccines in Hong Kong, (iii) the mobility data during the pandemic period, (iv) the social contact data in the community during the pre-COVID period, to simulate the number of infections that may be caused by the potential fifth epidemic wave under different interventions.
Assuming (i) 65% of the population being vaccinated with at least one dose of the vaccine, (ii) the vaccine effectiveness against infection as 50% (Sinovac) and 70% (BioNTech), (iii) the basic reproduction number (R0) of Omicron as 8 (when the entire population is susceptible, not vaccinated, or there is no exercising of social distancing measures) and (iv) antibody waning across time, (v) contact tracing efficacy similar to that in the fourth wave, and (vi) the severity rate of Omicron being half of that of Delta, the research team forecasts the upcoming epidemic based on these two scenarios:
If the social contact level remained at the same level as during the Christmas and New Year holidays in the past two weeks, the fifth epidemic wave triggered by Omicron would lead to more than 3 million infections, and more than 170 thousand severe cases.
If the tightened social distancing measures effective from 7 January would:
2.1. decrease the social mobility by 25% (daily number of social encounters =14), then the fifth epidemic wave would cause about 250 thousand infections and about 15 thousand severe cases;
2.2. decrease the social mobility by 40% (daily number of social encounters = 11), then the fifth epidemic wave would cause about 12.5 thousand infected cases and about 700 severe cases;
2.3. decrease the social mobility down to the trough in the pandemic (between July-August 2020, during the third epidemic, i.e., the social mobility decreases by 55% which corresponds to daily number of social encounters = 8) then the fifth epidemic wave would cause about 1,100 infections and about 50 severe cases.
Press release: Assessment of Omicron outbreak in Hong Kong after reimplementation of some of the toughest social distancing measures effective from 7 January
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