How well does the Oxford vaccine work? What we know so far

How the Oxford/AstraZeneca jab works, its advantages and possible limitations

What is the latest news?

The UK's Medicines and Healthcare products Regulatory Agency has approved the vaccine developed by Oxford University and the pharmaceutical company AstraZeneca, with the rollout beginning on January 4th.

How does this vaccine work?

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.

How well does it work?

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 a full dose, the vaccine had a higher efficacy than when participants were given two full doses, with 90 per cent efficacy in the former case and 62 per cent efficacy in the latter.

The results were met with both curiosity and caution – not least since the group given a half dose followed by a full dose was considerably smaller than the group given two full doses, and did not include participants aged over 55.


The controversy led AstraZeneca to announce in November a new global trial of the vaccine with the half dose/full dose regime.

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 per cent 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.

Is the Oxford/AstraZeneca vaccine really a “gamechanger”?

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.

While the Pfizer/BioNTech vaccine has already gained regulatory approval, and been administered to more than 600,000 people in the UK so far, it has posed a logistical challenge as the vaccine must be stored and transported at -70 degrees and can only be stored in regular medical refrigerators at 2 degrees to 8 degrees for up to five days.

By contrast, the Oxford/AstraZeneca vaccine can be transported and stored at 2 degrees to 8 degrees for up to six months, making it much easier to move around the country and administer in settings such as care homes, local pharmacies and prisons. It also makes it suitable for use in rural areas and countries where access to ultra-low temperature storage is a problem.

In addition, while the UK government ordered 40 million doses of the Pfizer/BioNTech vaccine, it has ordered 100 million doses of the Oxford/AstraZeneca jab, of which 4 million are available straight away, and 40 million expected by the end of March.

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.

But Prof Adam Finn, a vaccine expert at the University of Bristol and an investigator on the Oxford trial, said it was important to have a range of vaccines available, including those from Pfizer/BioNTech and Moderna.

“We need to find out what happens when we combine these different types of vaccine in the same person – perhaps we can get broader, stronger or more long-lasting immunity given one followed by the other,” he said, adding other kinds of vaccines were also in the pipeline, including some based on whole inactivated virus.

Prof Danny Altmann of Imperial College London agreed that several vaccines would be necessary, not least to ensure a large enough supply, but cautioned against comparing them at present, noting there was still much to learn about how the different vaccines performed.

“There is still an enormously steep mountain of vaccination to climb – nationally and globally – before any of us can feel safe and return to normal,” he said. – Guardian