The Oxford/AstraZeneca vaccine is based on a harmless chimp cold virus that cannot grow inside human cells. Scientists have tweaked this virus so that it carries genetic material containing the instructions for a protein of the coronavirus. Once the vaccine has been administered, our bodies produce the coronavirus protein, triggering an immune response.

That depends. Two doses of the vaccine, four weeks apart, are needed to offer the best protection against Covid. However, a dosing error led to the serendipitous finding that when clinical trial participants were given half a dose followed by full dose, the vaccine had a higher efficacy than when participants were given two full doses, with 90% efficacy in the former case and 62% efficacy in the latter.

However, researchers have stressed that regardless of which dosing regime was used, none of the trial participants developed severe Covid or had to go into hospital with the disease after having just one dose of the vaccine.

In addition, scientists say data from almost 24,000 trial participants showed only three serious safety events possibly related to a vaccine, at least one of which occurred in the control group, while both older and younger participants showed a similar immune response to the jab.

As with the mRNA vaccine from Pfizer/BioNTech, which has already been approved by the UK, US and EU, and which has 95% efficacy after two doses, it is currently unclear how long protection induced by the Oxford/AstraZeneca vaccine lasts.

And while there are some early signs the Oxford/AstraZeneca vaccine might not only prevent people becoming seriously ill with Covid but also prevent asymptomatic infections, more data is needed to confirm such protection.

The Oxford-AstraZeneca vaccine has been shown to be safe and to provoke an immune response in people of all ages, including the over-55s.

There are three figures doing the rounds - 62%, 70% and 90%.

The first analysis of the trial data showed 70% of people were protected from developing Covid-19 and nobody developed severe disease or needed hospital treatment.

The figure was just 62% when people were given two full doses of the jab and 90% when they were first given a half dose and then a full one.

The Medicines and Healthcare products Regulatory Agency (MHRA) has approved two full doses of the Oxford-AstraZeneca vaccine.

However, unpublished data suggests that leaving a longer gap between the first and second doses increases the overall effectiveness of the jab.

There was not enough clear data to approve the half-dose, full-dose idea.

All the vaccines are expected to be equally effective against the new variants of the virus that have emerged.

For years, Oxford researchers have been testing their chimpanzee adenovirus vaccine, ChAdOx1, on a number of other diseases including Ebola and Zika. Although none of those studies have reached the final, so-called Phase 3 trials, they have allowed researchers to examine the safety of the vaccine platform. The researchers have not found any serious side effects.

When the researchers adapted ChAdOx1 for Covid-19, their early clinical trials also did not turn up any adverse reactions.

In Phase 3 trials, however, the testing had to be paused twice when volunteers experienced neurological problems. The Food and Drug Administration did not directly tie the vaccine to the problems, but when the agency allowed the trial to resume in the United States, it advised the company to be vigilant for any signs of similar problems.

AstraZeneca and Oxford said that no serious safety issues were confirmed related to the vaccine.

The MHRA has authorised two full doses of the vaccine, with the second dose given four to 12 weeks after the first.

It said the vaccine was up to 80% effective when there was a three-month interval between the first and second doses.

A first dose of the jab gives about 70% effectiveness from three weeks after immunisation until a second dose at 12 weeks, according to the MHRA and the Joint Committee on Vaccination and Immunisation (JCVI).

The MHRA said it could not recommend a half-dose, full-dose regime for the Oxford vaccine, saying it was "not borne out" by the full analysis.

AstraZeneca’s vaccine has a number of advantages over other leading vaccine candidates: It’s easier to mass produce and store, and it’s also cheaper, at $3 to $4 per dose.

The Oxford/AstraZeneca vaccine is certainly expected to be a key component of the UK’s vaccination programme, and is the cornerstone of hopes for global vaccination.

The Oxford/AstraZeneca vaccine can be transported and stored at 2C-8C for up to six months, making it much easier to move around the country. It also makes it suitable for use in rural areas and countries where access to ultra-low temperature storage is a problem.

And there is a financial side: the Oxford/AstraZeneca vaccine costs about $3-4 per shot, compared with $20 for the Pfizer/BioNTech jab, with the former being made on a not-for-profit basis for the duration of the pandemic.

• https://www.nature.com/articles/d41586-020-03326-w
• https://www.theguardian.com/world/2020/dec/30/how-well-does-the-oxford-vaccine-work-what-we-know-so-far
• https://www.bbc.com/news/health-55280671