Human trials of the coronavirus vaccine candidate being developed at the University of Oxford are reported to be looking promising.

It is thought that early results indicate the jab could provide double protection – generating an immune response which stimulates the body to produce both antibodies and “killer T-cells”.

But what does the vaccine do, how does it work, and what happens next?

– What is the vaccine?

The vaccine – called ChAdOx1 nCoV-19 – uses a weakened version of a common cold virus (adenovirus) which causes infections in chimpanzees.

It has been genetically changed so it is impossible for it to grow in humans.

Researchers hope their version will make the body recognise and develop an immune response to the spike protein – recognisable in images of the virus – that will help stop Covid-19 from entering human cells and therefore prevent infection.

– What do the early results suggest?

While the researchers are not expected to publish the results of their Phase 1 clinical trials until July 20, reports indicate that the vaccine candidate has triggered two responses in the immune system.

The first is that it stimulates the immune system to produce antibodies – proteins produced by the blood in response to antigens which are harmful substances that come from outside the body, such as from viruses or bacteria – and that it also causes the body to produce T-cells.

If the non-specific immune cells which respond to any invader instantly cannot tackle it, the T-cells come into play.

They take two forms – helper T-cells and “killer” T-cells. The latter attack the virus directly.

With questions remaining about the duration of the antibody response to Covid-19, research suggests T-cells have a more important role in offering protection against the disease.

It is not yet known whether the Oxford vaccine candidate provides long-term immunity.

– How are the trials progressing?

More than 4,000 participants are already enrolled in the UK, with enrolment of a further 10,000 people planned as researchers test the ChAdOx1 nCoV-19 vaccine.

Trials are also taking place in South Africa and Brazil and it is hoped an effective vaccine could be ready later this year.

This trial aims to assess how well people across a broad range of ages could be protected from Covid-19.

It will also provide valuable information on safety aspects of the vaccine and its ability to generate good immune responses against the virus.

– Do enough people in the UK have Covid-19 in order for trials to show efficacy?

Professor Sarah Gilbert, who is leading the Oxford research, has said low transmission rates in the UK mean there is “little chance” of trials in the country proving the effectiveness of a coronavirus vaccine.

A sufficient number of volunteers have to be exposed to the virus to see whether a vaccine protects them or not.

However, if their chances of being in contact with an infected person are low, it will take a long time to demonstrate the efficacy of a vaccine candidate.

– What can be done to combat this?

Researchers have started trialling the vaccine in countries where there is a higher infection rate.

But some are calling for challenge trials, which involve deliberately exposing people to the virus after giving them the vaccine.

A number of prominent scientists, including Nobel laureates, are calling for volunteers to be exposed to coronavirus after receiving a vaccine to see if it offers protection.

They have signed an open letter to the head of the US National Institutes of Health (NIH), saying these challenge trials could accelerate vaccine development.

The signatories, who include Adrian Hill, director of the Jenner Institute at Oxford where the vaccine was developed, are calling on the NIH, its allies, international funders, and world bodies like the World Health Organisation, to undertake immediate preparations for human challenge trials.

They suggest this should involve supporting safe and reliable production of the virus and any biocontainment facilities necessary to house participants.

The authors write: “If challenge trials can safely and effectively speed the vaccine development process, there is a formidable presumption in favour of their use, which would require a very compelling ethical justification to overcome.”

In the letter the signatories also set out principles for a challenge trial, including an ethical and scientific review.

– If a successful vaccine is developed, can it be manufactured to scale?

Production of the vaccine has already been scaled up ahead of the trial to prepare as early as possible for potential future deployment.

AstraZeneca has reached an agreement with Europe’s Inclusive Vaccines Alliance (IVA) to supply up to 400 million doses of the University of Oxford’s Covid-19 vaccine at no profit, with deliveries starting by the end of 2020.

The British Government has agreed to pay for up to 100 million doses, adding that 30 million may be ready for UK citizens by September.