Trials involving 1,077 people showed the injection of the vaccine -called ChAdOx1 nCoV-19- led to them making antibodies and T-cells that can fight coronavirus.
Much of the focus on coronavirus so far has been about antibodies, which are small proteins made by the immune system that stick onto the surface of viruses. Neutralizing antibodies can disable the coronavirus.
T-cells, a type of white blood cell, help co-ordinate the immune system and can spot which of the body's cells have been infected and destroy them. Nearly all effective vaccines induce both an antibody and a T-cell response.
The results published on Monday in the Lancet medical journal are promising but preliminary.
The primary purpose is to ensure the vaccine is safe enough to give to people. More than 10,000 people will take part in the next stage of the trials in the U.K., as well as 30,000 people in the U.S., 2,000 in South Africa, and 5,000 in Brazil.
Prof. Sarah Gilbert, from the University of Oxford, told the BBC: "There is still much work to be done before we can confirm if our vaccine will help manage the Covid-19 pandemic, but these early results hold promise."