The Covid-19 vaccine being developed at the University of Oxford is safe and induces an immune reaction, “encouraging” preliminary results of the study suggest.

Researchers say their tests have revealed that the jab could provide double protection against coronavirus.

The early stage trial found that the vaccine is safe and causes few side effects.

While the current results focus on the immune response measured in the laboratory, further testing is needed to confirm whether the vaccine effectively protects against infection.

The findings have been widely welcomed with Boris Johnson tweeting that the trial results are “very positive news” but cautioning that there are no guarantees.

England’s chief medical officer, Professor Chris Whitty, tweeted: “This is encouraging news.

“A long way to go, but a testament to the remarkable scientific effort in the UK and around the world to combat Covid-19.”

The study found that the vaccine induces strong immune responses in both parts of the immune system – provoking a T cell response within 14 days of vaccination, and an antibody response within 28 days.

Compared with the control group of those given a meningitis vaccine, the Covid-19 vaccine caused minor side effects more frequently, according to the study.

But some of these could be reduced by taking paracetamol, the researchers said, adding that there were no serious adverse events from the vaccine.

Co-author Professor Sarah Gilbert, of the University of Oxford, said: “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.

“If our vaccine is effective, it is a promising option as these types of vaccine can be manufactured at large scale.

“A successful vaccine against Sars-CoV-2 could be used to prevent infection, disease and death in the whole population, with high-risk populations such as hospital workers and older adults prioritised to receive vaccination.”

Speaking to the PA news agency, Professor Teresa Lambe, associate professor and Jenner investigator, The Jenner Institute, University of Oxford, said the team was “thrilled” to be able to share the results.

When asked what she thought were the chances of the vaccine being successful, she added: “I’m not somebody who gambles very often so I’m not going to put a percentage on it.

“I’m hopeful that we will get a number of vaccines, and that we will get a number of manufacturing opportunities to deliver lots of vaccines that are effective across the world.

“So I don’t think that we can realistically look to one vaccine same to protect the whole world. I don’t think that’s, that’s too much of a pipe dream for me, I’m afraid.”

Phase two, in the UK only, and phase three trials to confirm whether it effectively protects against the virus are taking place in the UK, Brazil and South Africa.

The trial included 1,077 healthy adults aged 18-55 years with no history of Covid-19, and took place in five UK hospitals between April 23 and May 21.

The data included in the paper, published in The Lancet, covered the first 56 days of the trial and is ongoing.

The most commonly reported reactions were fatigue and headache, but some participants also reported pain at the injection site, muscle ache, malaise, chills, feeling feverish, and high temperature.

In addition, in the 10 people who received the extra dose of the Covid-19 vaccine, side effects were less common after the second dose.

The researchers say the vaccine might may entail an initial vaccination followed by a booster shot.

The research found that the vaccine stimulates an antibody and T-cell response.

Antibodies are 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.

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

Four weeks after vaccination, neutralising antibody responses against Sars-CoV-2 were detected in participants who received a single dose of the vaccine.

These responses were also present in all participants who had a booster dose of the vaccine.

It is not yet known how long immunity from Covid-19 lasts, but the vaccine developers say this does not mean their jab will not provide long-term immunity.

Prof Adrian Hill said it was very unlikely that any immunity to Covid-19 provided by the vaccine would only be short-term.

Speaking at a Science Media Centre, press briefing Professor Adrian Hill, director of the Jenner Institute for Vaccine Research: “Making the assumption that if natural infection doesn’t give you immunity for very long therefor a vaccine won’t give you immunity for very long – that doesn’t follow.”

He continued: “What matters is the type of vaccine technology you are using.

“The other upbeat response is that there aren’t really vaccines out there that just last for a few months – by and large vaccines last for some years, or at least a year and then you might need a top-up.

“(Immunity) is not going to disappear very quickly.”

The authors note a number of limitations to their study, saying more research is needed to confirm their findings in different groups of people, including older age groups, those with other health conditions, and in ethnically and geographically diverse populations.

In the current trial, 91% of participants were white and the average age of participants was 35 years.