On 31 December, while others were busy preparing for New Year parties, Sarah Gilbert found alarm bells ringing in her head. On the internet, there were reports of an outbreak of "viral pneumonia" in Wuhan, China.

"We'd been planning for Disease X," says the professor of vaccinology. "And I thought, this could be it."

By "Disease X", she meant a sinister new health threat that would take the world by surprise. She and her team at Oxford's Jenner Institute had worked out a strategy to produce a vaccine in the shortest possible time - and yesterday it was triumphantly vindicated.

Trials showed that a small dose of their vaccine followed by a larger one was 90% effective in protecting against Covid-19. For flu vaccines, even 60% is considered a good rate.

The vaccine, named ChAdOx1 nCoV-19, mimics the coronavirus and trains the body to react if the recipient later contracts it.

The Oxford team was able to develop it so quickly because they had already created a vaccine against MERS on the same principles. They had taken a virus that causes the common cold in chimpanzees and altered its genetic code so that it did not reproduce in humans. Once that had been done, it could be further modified to deal with whatever disease a vaccine was needed for.

In the case of Covid-19, the team added the genetic sequence of the "spike protein" - the key the virus uses to break into the body's cells. The vaccine could now train the immune system to recognise and attack that spike.

Even with so much prior expertise, the team had to work day and night to develop their new vaccine. By April, they were ready to start trials.

Normally the testing process takes years. But in an extraordinary feat of organisation, the vaccine has now been tested on 24,000 people in the UK, Brazil, and South Africa. None of them has developed a severe case of Covid-19.

Remarkably, Sarah Gilbert is an accidental vaccine expert. Her early research was into yeast for brewing, and she arrived at Oxford to work on a genetics project. That project just happened to be relevant to malaria, which sparked her interest in vaccines. She is now thought to be in contention for a Nobel Prize.

Two other successful vaccines have been announced, one by Pfizer/BioNTech and the other by Moderna. They are expected to be as much as 95% effective - but very expensive, costing £28 and £15 per dose respectively. Many worries that this could lead to "vaccine apartheid", in which rich countries receive the best while poorer ones rely on the leftovers.

But the Oxford vaccine could be a "vaccine for the world" rather than a "vaccine for the few". With the university's partner AstraZeneca producing it on a non-profit basis, it could cost as little as £3 a dose.

Furthermore, the Pfizer vaccine has to be stored at -70C, whereas Oxford's and Moderna's can be kept in ordinary fridges - and are the easiest to transport.

Some say, yes. It is highly effective, far more affordable than the alternatives, and easier to deliver - so for countries like India and Peru, it is the only real option. AstraZeneca has already taken orders for 3 billion doses and is geared up to produce and distribute it across the globe. It has been given geographically and ethnically diverse trials to make sure that it works for everybody.

Others argue that it is too early to say. There are many other vaccines still in development that could prove yet more effective and even cheaper. Russia's Sputnik V vaccine has not yet completed its trials, but results so far suggest that it is 92% effective. Two Chinese vaccines are also in the final testing phase, as is one produced by the global pharmaceutical company Janssen.