SCIENTISTS “can clearly see the end of tunnel now” in the battle against the Covid pandemic, after a third vaccine in as many weeks was shown to be up to 90 per cent effective in preventing the disease.

Researchers leading the Oxford University-AstraZeneca vaccine trial, which has involved 20,000 volunteers in the UK and Brazil, reported that their vaccine protected more than 70% of people from becoming ill after two full doses.

The results are based on comparing rates of naturally acquired Covid infections in participants who have been vaccinated against those in a placebo group, who received a standard meningitis jag.

In a twist, however, the results were even stronger - up to 90% protection - when participants were given a half dose first, meaning that even more people could be immunised with existing stocks.

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AstraZeneca has already manufactured the equivalent of four million doses, which would be ready to deploy as soon as licensing is approved.

The findings come hot on the heels of breakthroughs with the Pfizer-BioNTech and Moderna Covid vaccines, which have each reported up to 95% efficacy.

It is unprecedented in medicine to have multiple different vaccines available against a single disease, and several others are still end final-stage trials

If all three pass safety and efficiency tests by regulators, it would put the UK in the position of having more vaccines than would actually be required to inoculate the entire population in a first round - although booster jags may be required in subsequent years depending on how long immunity lasts.

The UK pre-ordered 100 million batches of the Oxford vaccine - potentially enough to vaccinate 150 million people - and has also ordered 40 million doses of the Pfizer-BioNTech vaccine, which would be enough to immunise 20 million with two doses three weeks apart. Both are expected to be rolled out in December pending regulator checks.

The UK has also ordered five million doses of the Moderna vaccine, which should be delivered by spring next year.

The Oxford vaccine has been hailed by experts as particularly significant because it is much cheaper and easier to store than the other two vaccines.

It costs £3 compared to £15 for the Pfizer vaccine and £25 for the Moderna vaccine, which each require two full doses.

The Oxford vaccine can also be preserved in the fridge, meaning it can be rolled out more easily worldwide.

The Pfizer-BioNTech vaccine in particular must be transported at -70°C - colder than Antarctica in winter - and will only last up to a week in a GP fridge, while the Moderna vaccine must be kept at -20°C prior to use, roughly equivalent to a domestic freezer.

As well as preventing serious illness, the researchers said there was “just a hint in the data” that those in the one and a half dose cohort were also prevented from developing asymptomatic Covid infections.

Professor Andrew Pollard, director of the Oxford Vaccine Group and chief investigator of the trial, said: “If that is right, we might be able to halt the virus in its tracks and stop it transmitting between people.”

Peter Horby, professor of emerging infectious diseases and global health at Oxford University, said the news of a third effective vaccine means “we can clearly see the end of tunnel now”.

He added: “Importantly, from what we have heard the vaccine seems to prevent infection not just disease. This is important as the vaccine could reduce the spread of the virus as well as protect the vulnerable from severe disease.”

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No Covid hospitalisations or deaths were reported among any of the participants who received the Oxford vaccine, and no serious adverse reactions occurred although the trial is ongoing.

Full safety data will have to be submitted to regulators and, as with the other two Covid vaccines, the evidence behind them has not yet been peer-reviewed by other scientists.

Health Secretary Jeane Freeman has said the NHS in Scotland would be ready to vaccinate people from the first week of December if safety approval is given, with a target to inoculate up to one million people by the end of January if there are no delays.

Care home residents and staff will be prioritised under guidelines drawn up by the UK’s Joint Committee on Vaccination and Immunisation (JCVI) .

Frontline NHS workers and the over-80s will be offered the vaccine next, before it is rolled out to over-65s and then under 65s at high clinical risk.

It is impossible to know at this stage what level of uptake will be required to achieve herd immunity in the population, but in general the more effective a vaccine is the lower the threshold.

Even an effectiveness rate of 70%, however, would exceed the protection delivered by seasonal flu vaccinations which save millions of lives worldwide each year.

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Dr Gregor Smith, Scotland’s interim chief medical officer, said the initial indications for the Oxford vaccine are “very positive and encouraging”.

“What we see is evidence of high efficacy of this vaccine using two different dose regimes and high safety profiles as well,” said Dr Smith.

“We can’t but be encouraged by that, particularly as the availability of this vaccine is likely to be fairly imminent.”

Professor Deborah Dunn-Walters, chair of the British Society for Immunology Covid-19 and Immunology taskforce, said it was “intriguing” that better results were reported in the group receiving a half dose first followed by a full dose four weeks later.

One theory is that efficiency decreases at two full doses because the body recognises and mounts “an anti-vector immune response to the second dose, which is lower if the first dose is reduced”.

The Oxford vaccine has been created using a weakened adenovirus which normally causes the common cold in chimpanzees.

This harmless “vector” carries the genetic blueprint for the SARS-CoV-2 spike protein into the human cells, to prime an immune response against Covid.

By contrast, the Pfizer and Moderna vaccines use cutting edge synthetic mRNA technology, instead of a vector.

Peter Openshaw, professor of experimental medicine at Imperial College London, said: “The report that an initial half-dose is better than a full dose seem counterintuitive for those of us thinking of vaccines as normal drugs: with drugs, we expect that higher doses have bigger effects (and more side-effects), but the immune system does not work like that.

“The report that an initial half-dose then a full dose is even better than two full doses is great news, potentially increasing the number of people that can be vaccinated and reducing costs.”