ITV Science Editor Tom Clarke answers some questions about the AstraZeneca vaccine
"Thrombosis with thrombocytopenia" - it's a bit of a mouthful, it's a complex and poorly understood syndrome and it's also the reason why regulators have decided to recommend an alternative vaccine to the Oxford/AstraZeneca jab in people under the age of 30. In new data released by UK medicines regulator the MHRA on Wednesday, there have now been 79 cases of the syndrome and 19 deaths among 28 million doses of the vaccine given in Britain.
That equates to around four cases for every one million people vaccinated with the AstraZeneca jab.
Exactly why, and why it's seen just with the AstraZeneca vaccine and not other Covid vaccines - which are all based on the same "spike protein" part of the coronavirus - remains a mystery.
The syndrome has been reported in people aged from 18 to 79. Of those, 51 were women and 28 were men. There is some evidence the risk of the side effect decreases as people get older. However, the main reason for drawing a line at age 30 and below relates not to a person's increased risk of the side-effect - it is after all exceedingly rare - it's to do with the balance of risk and benefit of having the vaccine.
Regulators have concluded that above the age of 30, the risk of serious harm or death from Covid is higher than the very small risk of a potentially fatal blood clot. Below the age of 30, the risk of dying from Covid, if infected, is less than one in 2,500. And of course, the risk is lower still because not everyone gets infected.
Video report by ITV Science Editor Tom Clarke
This fact shifts the balance of risk and benefit to the point where it tips away from using the vaccine in this age group. The announcement is a significant blow to AstraZeneca and the University of Oxford team that developed the vaccine.
It could also further undermine confidence in the AstraZeneca jab which has been beset by criticism - much of which turned out to be false - since its launch.
But it could also have ramifications for the global fight against Covid.
The AstraZeneca vaccine is made on a not-for-profit basis and is one of the most affordable and available vaccines for developing countries via the World Health Organization's Covax initiative.
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Given populations in many developing countries are significantly younger than in the developed world, it's possible the UK and European regulator's decisions could be a significant setback.
When I asked him about the MRHA's decision, Professor Wei Shen Lim, Covid-19 chair of the Joint Committee on Vaccination and Immunisation (JCVI), said: "For somebody who is 31 or 32, I think they have to make their own decision as to what they want to do about vaccination.
"We would still say that the balance is in favour of being vaccinated because of the risk of Covid and the protection that the vaccine offers."
So what is thrombosis with thrombocytopenia?
Thromboses, or blood clots, are common. They can form in the legs and move to the lungs (deep vein thrombosis leading to pulmonary embolism) or elsewhere in the body.
When it comes to the vaccine-related syndrome however, the type of blood clot is rarer.
The most commonly seen event has been a serious blood clot called cerebral venous sinus thrombosis (CVST) – a specific type of clot in the main vein running down the centre of the brain which prevents blood from draining. But it's more than just that. Most blood clots are caused by things called platelets in the blood clumping together.
These rare clots are occurring with very low levels of platelets in the blood -- thrombocytopenia.
It makes managing the syndrome difficult and specialists who have treated it say they are still having to "feel their way forward" in managing each case. The fact it is very rare is one of the main reasons it took regulators weeks to come to this conclusion after the initial alarm was raised.
Without a compelling explanation for a link, the numbers of events were still below the level at which they could have been occurring by chance. Another complicating factor has been the fact Covid infection itself causes blood clots.
The Prime Minister said he doesn't see any reason to deviate from the roadmap out of lockdown:
Regulators wanted to be certain that it wasn't undiagnosed, or a previous Covid infection causing blood clots.
Recent data suggests 30-70% of patients with Covid on intensive care units develop blood clots in the deep veins of the legs or in the lungs. It's an important fact anyone should consider if being offered the AstraZeneca vaccine.
Wednesday's change to vaccination advice is being made to limit reduce the chance of a very rare side effect.
For those over 30, the risks of Covid aren't rare at all.