Covid: Is there a blood clot risk with the Oxford/AstraZeneca coronavirus vaccine?

Tom Clarke

Former Science Editor

A medical trial has been paused among children recieving the AstraZeneca vaccine. Credit: PA

The AstraZeneca Covid vaccine is back in the news again.

On Tuesday evening, a trial of the vaccine in around 300 children was paused while the UK medicines regulator investigates the possibility of links to a rare blood clotting syndrome in adults.

In a statement the University of Oxford that is leading the trial said: “Whilst there are no safety concerns in the paediatric clinical trial, we await additional information from the MHRA [the UK's Medicines and Healthcare products Regulatory Agency]... before giving any further vaccinations.”

The announcement comes as the UK and European drug watchdogs are expected to publish their latest findings into a possible link between the AstraZeneca jab and blood clots.

On Tuesday, a senior official with Europe’s drug watchdog, the European Medicines Agency (EMA) told an Italian newspaper: “In my opinion we can now say it is clear that there is an association with the vaccine [and a rare form of blood clot].”

  • Science Editor Tom Clarke explains latest AstraZeneca fallout

The EMA later said no decision had yet been made but that an announcement was forthcoming.

The UK medicines regulator (MHRA) is also expected to publish new data on a possible link.

At issue is not just whether or not the vaccine causes a rare form of blood clots, but whether the risk of this possible side effect outweighs the benefit of having the vaccine.

Answering the first question is hard, the second, even harder.

First things first: the blood clot issue. There certainly seems to be something going on.

Haematologists — blood doctors to you and me — became concerned when they started seeing blood clots in people with very low levels of platelets in their blood.

This is something of a medical oddity because platelets are the thing in the blood that cause blood clots. But not unheard of — it’s just very, very rare. And that’s what caused raised eyebrows. Normally you’d only expect to see a few cases of such a syndrome in any given year.

Then haematologists from across Europe began to learn their colleagues had been reporting higher than expected numbers of cases following vaccination with the AstraZeneca Covid jab.

That’s when the regulators were warned that there might be an issue.

Prime Minister Boris Johnson visited the AstraZeneca site in Macclesfield, Cheshire Credit: PA

The cases are still very rare — the UK has seen 30 out of 18 million AstraZeneca doses given, Germany 30 cases out of 2.7 million.

Assuming the vaccine IS causing this blood clotting syndrome (and that remains an assumption) the risk is small: One in 600,000 (based on UK data) or one in 90,000 (based on German data).

The two figures show a big difference in risk — possible explanations are Germany has vaccinated more younger people with the AZ vaccine than the UK or alternatively, there is no link between the vaccine and the syndrome.

In response the British Society for Haematology has published guidance for doctors who may come across cases of the syndrome.

But given the occurrence is rare, it’s very difficult to prove with statistical certainty that the syndrome is caused by the vaccine.

It’s why the regulators continue to advise that the vaccine’s benefits outweigh the risks.

Even the haematologists who first identified the potential problem say the same.

When will we know if the effect is real?

Based on the most recent data (up until March 25) there still isn’t enough evidence to say that it is. However, as time passes more and more doses are being given and the statistics become clearer.

Germany is restricting the use of the AstraZeneca jab to over 60s. Credit: PA

There are lots of reasons to be sceptical beyond the very low incidence of the blood clotting syndrome.

Why is it more prevalent in younger people than in older individuals?

Typically older people are more vulnerable to clotting.

Why is this being reported with the AstraZeneca vaccine but not with nearly identical Sputnik V and SinoVac jabs?

Some cases of the blood clotting syndrome are being reported four days after vaccination — according to vaccine experts it typically takes much longer for similar adverse immune responses to manifest themselves.

It’s anticipated that the current review by the regulator should bring more certainty.

But even if it does confirm a connection between the AstraZeneca vaccine, what then?

It then becomes a complicated question of the benefits of the vaccine and the risks of the side effect.

This has a lot to do with how old you are.

The younger you are, the lower your risk of dying from Covid so the risk of the vaccine (if there is any) becomes far more significant.

It also depends on how bad the current outbreak is.

If your chance of getting infected is lower, the relative risk/benefit balance changes too. And while the risk of a younger person dying of Covid is low, the rates of “long Covid” are high in younger adults.

If a risk of blood clots is proven, and to reiterate, it is still an IF, the medicines regulator has to make a decision.

Does it restrict the use of the vaccine in younger adults — and if so, what age?

Or does it recommend any age group can have it, but there is a warning on the label about a very rare side effect? This again is a balancing act.