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Covid: How does the Oxford-AstraZeneca vaccine work?


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Discussions about a possible link between the Oxford-AstraZeneca vaccine and rare blood clots are continuing.

UK Prime Minister Boris Johnson says people should listen to medical regulators and keep getting their jabs. Some EU countries recently paused the rollout of the vaccine amid concerns.

What is the advice about blood clots?

Regulators are carrying out a review into reports that a very small number of recently-immunised people suffered an extremely rare form of blood clot, called a cerebral sinus vein thrombosis (CSVT).

In the UK, 30 people have developed the clots – and seven have died as a result – out of 18 million people who have received the vaccine.

The clot can occur naturally, and no link to the vaccine has been established.

Currently, the official position of both the UK regulator and the European Medicines Agency (EMA) is that the vaccine is safe and there is no evidence that the vaccine causes clots.

As a precaution, regulators will continue to monitor the situation and say symptoms such as unusual bruising or a persistent headache after vaccination should prompt a medical check.

Covid infection can be serious, and can itself make clots more likely.

How does this vaccine work?

It is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees. It has been modified to look more like coronavirus – although it can’t cause the illness.

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

Is the Oxford vaccine as good as the Pfizer?

Large trials showed the Pfizer vaccine was 95% effective, while the figure for the Oxford one was 62%.

But directly comparing results is difficult because there are differences in the way the trials were carried out.

No-one who received the Oxford vaccine was hospitalised or became seriously ill due to Covid.

A recent study found a single dose of the Oxford vaccine offered 76% protection for three months, and this went up to 82% after the second dose.

Does it work in older people?

Some European countries initially offered the vaccine only to 18-64 year-olds because they said there was limited data on how well it protects the over-65s.

Both Germany and France then reversed this stance, and recommended the vaccine for over-65s.

The EMA approved the vaccine in January for use in all age groups, including older adults.

How long do vaccines protect against Covid?

It is not yet known how long protection lasts with any of the coronavirus vaccines.

A study found that unvaccinated people who have had Covid, develop protection for at least six months. Vaccines are likely to provide stronger protection than this.

It may be that people need annual vaccinations, as happens with the flu jab.

Which vaccine will I get?

You will not be given a choice about which vaccine you get.

In the UK, recommendations on which groups get the vaccine are made by the Joint Committee on Vaccination and Immunology.

Does the vaccine protect against new variants?

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 for other ones, such as the Brazil and the South Africa variants.

South Africa has paused roll-out while it investigates further.

However, there is no evidence to suggest it would not be effective at preventing more severe cases that need hospital treatment.

UK scientists are working on new versions of the vaccine, to keep up with a virus that will inevitably keep mutating.



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