Oxford's COVID-19 vaccine clinical trials began in April 2020. The main focus of the Phase I, II and III studies is to assess whether the ChAdOx1 nCoV-19 vaccine is going to work against COVID-19, if it won’t cause unacceptable side effects and if it induces good immune responses.

Phase I/II

1,077 participants were recruited beginning in April 2020 across multiple study sites in Oxford, Southampton, London and Bristol. Participants were randomly allocated to receive either the ChAdOx1 nCoV-19 vaccine or a licensed vaccine (MenACWY) that is used as a ‘control’ for comparison. The dose used in this trial was chosen based on previous experiences with other ChAdOx1 based vaccines. 

There was also a separate small group of 10 volunteers who received 2 doses of ChAdOx1 nCoV-19 four weeks apart.

Study participants will not know whether they have received the ChAdOx1 nCoV-19 vaccine until the end of the trial.

After receiving the vaccine, participants record any symptoms experienced for 7 days. They also record if they felt unwell for the following three weeks. 

Following the vaccination visit, participants attend a series of follow-up visits to check their observations, and take a blood sample. These blood samples are used to assess the immune response to the vaccine, by testing neutralising antibody and T cell responses. 

Phase II/III

In total this study will enrol up to 10,560 adults and children across the UK.

The phase II part of the study involves expanding the age range of people the vaccine is assessed in, to include a small number of adults and children:

  • Aged 56–69
  • Aged over 70
  • Aged 5–12 years

For these groups, researchers are assessing the immune response to the vaccine in people of different ages, to find out if there is variation in how well the immune system responds in older people or children.

The phase III part of the study involves assessing how the vaccine works in a large number of people over the age of 18. This group will allow assessment of how well the vaccine works to prevent people from becoming infected with COVID-19. As of mid-July 2020, over 8,000 people had been recruited.

Following the same protocol as above, adult participants are randomised to receive one or two doses of either the ChAdOx1 nCoV-19 vaccine or a licensed vaccine (MenACWY) that is used as a ‘control’ for comparison.

To recruit the large number of participants needed for this trial, multiple clinical research sites across the UK are involved in delivering the study. This is a collaborative effort led by the University of Oxford and a full list of our study sites is available on the trial website.

International studies

In addition to the UK-based trial, phase III studies have now begun in Brazil, South Africa and the USA to assess the vaccine in other populations, with a related trial in India. The data is examined collectively across all official trial sites, and any cases of COVID-19 are recorded to assess the safety and effectiveness of the vaccine.


If participants develop COVID-19 symptoms during the study, they can contact a member of the clinical team, to check whether they have become infected with the virus.

To assess whether the vaccine works to protect from COVID-19, the statisticians in our team will compare the number of infections in the control group with the number of infections in the vaccinated group.

How quickly we reach the numbers required will depend on the levels of virus transmission in the community. With the current low transmission levels in the UK, this could take many months.

To increase the chance of achieving an efficacy result sooner, we are prioritising those who have a higher chance of being exposed to the SARS-CoV-2 virus, such as frontline healthcare workers, frontline support staff and public-facing key workers. Conducting studies in multiple locations where transmission levels vary also increases the chances of reaching an efficacy endpoint sooner.

The University of Oxford's vaccine development work

Our vaccine work is progressing quickly. To ensure you have the latest information or to find out more about the vaccine trial, please check our latest COVID-19 research news or visit the COVID-19 vaccine trial website.

The University of Oxford's vaccine development work
Close up of syringe and vaccine vial

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