Cobovax study
Randomized trial of COVID-19 booster vaccinations [Cobovax study]
Study Introduction
This is an open-label clinical trial on COVID-19 vaccine booster in Hong Kong organized by the School of Public Health, the University of Hong Kong. Hong Kong citizens aged 18 or above who have already received two doses of BioNTech or Sinovac at least 6 months ago are invited to participate in this research study. We will randomly allocate each eligible participant to a third dose of mRNA vaccine (BioNTech) or inactivated vaccine (Sinovac), allowing us to compare the differences between mix-and-match versus same-technology third doses, for example in terms of enhancing immunity against COVID-19 or reactions after vaccination. Then, we will track these participants for a period of 1 year (2021 – 2022) to monitor the post-vaccination reactions and immune protection after receiving COVID-19 booster vaccination.
這是一項在香港本地進行的有關新冠病毒(COVID-19)疫苗加強劑的開放標籤臨床研究計劃,由香港大學公共衞生學院主辦,邀請香港18歲或以上、在6個月前已完成接種兩劑復必泰或科興疫苗的市民參加。本研究計劃會為合資格參與者隨機接種第三劑mRNA疫苗(復必泰)或滅活疫苗(科興),以比較接種混合類型疫苗對比接種同類型疫苗在如提升抵抗新冠肺炎免疫水平或接種疫苗後反應的差異。接著,我們將追踪參加者1年 (2021 – 2022),以全面監測接種新冠疫苗加強劑後的接種反應及免疫保護。
Enrolment Eligibility
You may be eligible to join the study if you meet the following criteria:
  • 18 years old or above
  • Have received two doses of BioNTech (Comirnaty) OR two doses of Sinovac (CoronaVac), with the most recent dose at least six months (180 days) prior to enrolment
  • Willing to provide blood samples
You are not eligible to join the study if you meet any of the following criteria:
  • A history of confirmed COVID-19 infection
  • Have received any type of COVID-19 vaccines other than Comirnaty or CoronaVac before
  • You received the previous first and second dose of COVID-19 vaccine 43 days or more apart
  • Have allergies to the active substance or other ingredients of mRNA or inactivated COVID-19 vaccines
  • Have any medical condition related to the immune system as determined by a clinician
  • Use of medication that impairs immune system in the last 6 months, except topical steroids or short-term oral steroids (course lasting 14 days or less)
  • Have used any blood products within the past 90 days
  • Currently pregnant, or planning for lactation or to become pregnant in the coming 3 months
During the online registration and subsequently before enrolling to the study, our team will have more detailed assessment with you to see whether you are suitable to join this study.
  • 18歲或以上人士
  • 曾在至少6個月(180天)前接種了兩劑復必泰或科興疫苗
  • 願意提供血液樣本
  • 曾確診感染新冠病毒(COVID-19)
  • 曾接種除了復必泰或科興疫苗以外的任何其他新冠疫苗
  • 您已接種的第一劑及第二劑新冠疫苗相隔43日或以上
  • 對mRNA新冠疫苗或滅活新冠疫苗之活性成分或其他成分過敏
  • 有被醫生診斷與免疫系統相關的任何醫療狀況
  • 在過去6個月內曾使用抑制免疫系統的藥物,外用類固醇或短期口服類固醇(療程持續14天或以下)除外
  • 在過去90天內曾使用任何血液製品
  • 正在懷孕,或計劃於未來3 個月內懷孕或哺乳
COVID-19 is caused by infection with the SARS-CoV-2 coronavirus, which has caused millions of deaths and major economic impacts worldwide since the beginning of 2020. Since 2021, different countries or regions around the world including Hong Kong have been offering free COVID-19 vaccination to the public with different types of COVID-19 vaccines. Due to differences in vaccine design and composition, these different COVID-19 vaccines could be different in terms of the ability to stimulate different branches of immune responses.
The emergence of variants of concern (VOCs) and observed decrease in vaccine-induced immune responses within few months after vaccination have suggested the potential need for booster vaccination in the near future. However, so far there are only very limited data on the immunogenicity and safety of a booster (third) dose of COVID-19 vaccination. Also, only limited studies evaluate the use of a different vaccine platform as a 3rd “booster” dose to a homologous 2-dose regime. However, this regime could have advantages – research suggest that using mRNA vaccine as a booster to inactivated vaccines may enhance the immune response when compared to using inactivated vaccine as the booster and, increase vaccine effectiveness; and could also allow a flexible immunisation deployment given distribution challenges of vaccines at a global scale. Therefore, there is an urgent need to conduct this research to help scientists design better vaccine deployment strategy against the emerging VOCs.
This study has been approved by the Institutional Review Board of the University of Hong Kong/ Hospital Authority Hong Kong West Cluster (UW21-492).
新冠病毒「值得關切的變異株」(VOCs)的出現,及數據顯示接種疫苗後的免疫水平在數月內下降,反映短期內有可能需要考慮接種疫苗加強劑。然而到目前為止,關於接種COVID-19 疫苗加強劑(第三劑)的免疫原性和安全性的數據非常有限。再者,只有少數研究評估以基於不同設計的疫苗作為同源兩劑(homologous 2-dose)疫苗的第三(加強)劑。然而,這種方案有其優勢 – 研究提出以mRNA疫苗作為滅活疫苗的加強劑可進一步提升疫苗的免疫原性,並從而提高其疫苗有效性;及提供靈活的免疫接種計劃以面對疫苗供應全球性的挑戰。因此,我們有迫切需要進行是項研究,幫助科學家設計針對新發變異株更好的新冠疫苗接種策略。

If you are 18 years old or above, have received two doses of Sinovac (CoronaVac) or BioNTech (Comirnaty) 180 days ago, and interested to receive a 3rd booster dose of either an inactivated vaccine (CoronaVac) or a mRNA vaccine (Comirnaty) now

complete a simple online registration form
(Click “Join Now” )


Research personnel conduct initial screening on enrolment eligibility


Research personnel arrange a face-to-face interview with you for further assessment






During the interview, research personnel explain the details of the study to you and confirm your participation


Upon enrolment to the study, you will be randomised to receive either a (booster) dose of inactivated vaccine (CoronaVac) or mRNA vaccine (Comirnaty)


During the study period, regular follow-up activities include questionnaires and blood collection





No cost will be incurred for joining this study. To appreciate your support for our study, you will receive tympanic thermometer/ supermarket coupons during each regular follow-up for sample collection. In addition throughout the study, we may occasionally send you the results of your laboratory testing or findings of the study as a whole.


How to Join
  • Click “Join Now” to complete a simple registration form online
    • Due to popular demand, we aim to complete initial screening on enrolment eligibility and get back to you within two weeks after registration
  • Call our study hotline during office hours
  • 按此「立即參加」進行簡單網上登記
    • 報名反應熱烈,我們目標兩星期內回覆參於資格初步篩選結果
  • 於辦公時間內致電本研究熱線

Contact the research team

Cobovax study

School of Public Health, The University of Hong Kong

Phone/WhatsApp: +852 9446 3186


Office hours: Monday to Friday, 10am to 5pm (except public holidays)




電話 / WhatsApp:+852 9446 3186







The Science

About Coronavirus Disease 2019 (COVID-19)

The recent emergence of the new coronavirus disease 2019 (COVID-19) in late 2019 and the subsequent global pandemic has since caused millions of deaths and significant economic impacts globally. COVID-19 is caused by the infection of SARS-CoV-2 coronavirus where disease severity varies widely, from asymptomatic infection, mild self-limiting disease with common respiratory symptoms, to severe disease requiring hospitalization that may lead to long-term symptoms or death.

About COVID-19 Vaccine

COVID-19 vaccination could provide protection against SARS-CoV-2 infection or severe disease after infection, and would also help to reduce community transmission by the accrual of population immunity. Due to different vaccine formulations, different COVID-19 vaccines such as inactivated virus vaccines (e.g. Sinovac, Sinopharm), mRNA vaccines (e.g. BioNTech, Moderna) and non-replicating adenovirus-vector-based vaccines (e.g. Oxford–AstraZeneca, Johnson & Johnson, Sputnik V) may have different capacities to stimulate different branches of immune responses (e.g.  humoral, cell-mediated and innate immunity) and require different vaccination frequencies. Like other vaccines, mild allergic reactions (side effects) such as mild fever, swelling or pain at injection site, or, in rare cases severe allergic reactions, may occur after COVID-19 vaccination.
For more information about COVID-19 vaccines, please visit the official website of the Hong Kong government’s COVID-19 Vaccination Programme.

About vaccine booster shots

When we say “booster shots” for the current COVID-19 vaccination program, most usually refer to getting a third dose after an individual has received two prior doses of COVID-19 vaccination. Some reasons that may prompt individuals to consider receiving a third dose of COVID-19 vaccine include:
  1. in some more vulnerable groups such as immuocompromised individuals, compared to otherwise healthier individuals, the previous two doses of vaccination have not activated strong enough protective immune responses
  2. in any individuals, immune responses after vaccination may decrease over time naturally
  3. some COVID-19 vaccines may perform less well against new emerging virus strains
Preliminary data suggest the use of a same-technology (homologous) or a different-technology (heterologous) third dose using either Comirnaty or CoronaVac are safe, and data from other vaccine platforms suggest reactions after the third dose may be even less than that after the first dose. It is foreseeable that the administration of mRNA vaccine as the (heterologous) booster to inactivated vaccine may improve the vaccine immunogenicity when compared to vaccination with inactivated vaccines alone, or may even provide more advantage when compared to using inactivated vaccine as the (homologous) booster. Conversely, one may consider receiving an inactivated vaccine as a booster to (the primary series of) mRNA vaccines for presumably less post-vaccination reactions.
For further discussion on the need for COVID-19 vaccine boosters, please visit this page from the World Health Organisation (English only).






  1. 在一些較脆弱的群體中如免疫力較弱人士,與其他更健康人士相比,在接種前兩劑新冠疫苗後可能並未能產生足夠強的免疫保護反應
  2. 在任何人士,接種疫苗後的免疫反應可能會隨著時間自然地下降
  3. 一些新冠疫苗對抵抗新出現的新冠病毒株的效用可能較差
About Us
Our team at the School of Public Health, the University of Hong Kong are experts in infectious disease epidemiology. This study is led by Prof. Benjamin Cowling and Dr. Nancy Leung.
The School of Public Health, LKS Faculty of Medicine of The University of Hong Kong (HKUSPH) has a long and distinguished history in public health education and high impact research. With world leading research in infectious diseases as well as on non-communicable diseases of both local and global importance, we have made significant contributions through its research and advocacy to improve the health of populations and individuals, both locally and globally. The School is a leading research and teaching hub in public health on influenza and other emerging viruses, control of non-communicable and infectious diseases, tobacco control, air pollution, psycho-oncology, behavioural sciences, exercise science, life-course epidemiology, and health economics, health services planning and management. Work done by us has informed international (e.g. the World Health Organization and the Food and Agriculture Organization), national and local public health policies.
The School of Public Health hosts the WHO Collaborating Centre (WHO CC) for Infectious Disease Epidemiology and Control. With a view to protecting the public's health in Hong Kong and across the region, the WHO CC aims to coordinating research on the control and prevention of infectious diseases and providing local and regional education and training in infectious disease epidemiology and control. Members of the WHO CC are involved in the response to COVID-19 and conducted a range of scientific research projects. The team has created a website (click here to visit) to share the latest scientific findings and the implications for evidence-based public health policies on the ongoing COVID-19 pandemic.