Covid: How does the Oxford-AstraZeneca vaccine work?

People under the age of 30 are to be offered an alternative to the Oxford-AstraZeneca vaccine, after a review into a possible link with extremely rare blood clots in adults.

However, the UK's medicine regulator - the MHRA - says the benefits of the vaccine still outweigh the risks for the vast majority of people.

The MHRA looked into UK cases of rare blood clots in people who had recently received the Oxford-AstraZeneca vaccine.

It found that 79 people - two-thirds of them women - experienced clots after receiving a first vaccine dose. Nineteen of them died.

More than 20 million AstraZeneca vaccines doses had been administered across the UK by the end of March.

The MHRA said about four people in a million would normally be expected to develop this particular kind of blood clot - though the fact they are so rare makes the usual rate hard to estimate.

And the regulator said it had not been proven that the jab had caused the clots.

Its head, Dr June Raine, said while the link was "firming up", more evidence would be needed.

The benefits of the AstraZeneca vaccine outweigh the risks of the virus - hospitalisation and death - for the vast majority of people, she said. But for younger age groups it was more "finely balanced".

The MHRA study looked at people who had developed clots associated with a low level of platelets after receiving the Oxford-AstraZeneca vaccine.

Platelets are tiny blood cells that help your body form clots to stop bleeding.

Among these clots is a type called a Cerebral Venous Sinus Thrombosis (CSVT).

CVST occurs when a blood clot forms in large veins in the head - stopping blood from draining out of the brain.

As a result, blood cells may break and leak into brain tissue - ultimately leading to a stroke.

The clot can occur naturally and are more common, but still very rare, in younger women.

The MHRA says anyone who has these symptoms four or more days after receiving the Oxford-AstraZeneca vaccine should seek prompt medical advice:

Covid infection itself can make clots more likely, stresses the MHRA.

The MHRA is not recommending age restrictions regarding use of the AstraZeneca vaccine - but it says under-30s with no underlying health conditions should be offered an alternative vaccine where available.

This is because the data suggests there's a slightly higher incidence of clots reported in younger adult age groups.

Based on the current data, the following is also advised:

A trial of the Oxford-AstraZeneca vaccine on children had stopped giving out jabs while the MHRA carried out its review.

The European Medicines Agency (EMA) has said clotting should be listed as a very rare vaccine side effect.

After a study looking at 86 such cases in the EU, the EMA concluded the benefits of the vaccine outweighed the risk and that there was no definite causal link.

Nevertheless, Germany, Spain and Italy have suspended use of the vaccine in people under 60, while France recommends it only be given to those aged 55 or over.

Denmark has suspended its use.

It is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees. It has been modified to contain genetic material shared by the coronavirus - although it can't cause the illness.

Once injected, it teaches the body's immune system how to fight the real virus.

Experts are studying all of the current coronavirus vaccines to check how well they work against new, mutated variants of the virus.

The government's deputy chief medical officer, Jonathan Van Tam, says there is "plenty of evidence" the vaccines appear to be effective against the Kent variant which is dominant in the UK.

There is less evidence about protection against other variants, such as those identified in Brazil and South Africa.

However a study of around 2,000 people suggests, while the Oxford-AstraZeneca vaccine may offer more limited protection against mild and moderate disease caused by the South Africa variant. it should still protect against severe disease.