The UK's Medicines and Healthcare products Regulatory Agency (MHRA) has concluded that there is a possible link between AstraZeneca's COVID-19 vaccine and “extremely rare, unlikely to occur blood clots”.

In a statement, the regulator said the benefits of vaccination continue to outweigh any risks but it advised that careful consideration be given to people at higher risk of specific types of blood clots because of their medical condition.

Up to and including March 31, the MHRA had received 79 UK reports of blood clotting cases alongside low levels of platelets following the use of AZ' vaccine, which all occurred after the first dose. Nineteen people died as a result, of which 13 were females and six males.

Eleven out of the 19 who died were under the age of 50, three of whom were under 30. Fourteen of these 19 cases were of cerebral venous sinus thrombosis (CVST) with thrombocytopenia and five were of thrombosis with thrombocytopenia.

By March 31, 20.2 million doses of AZ' COVID-19 vaccine had been given in the UK, putting the overall risk of these blood clots at around four people in a million who receive the vaccine, according to the regulator.

The data suggest a slightly higher incidence reported in the younger adult age groups and so the MHRA advises that this evolving evidence should be taken into account when considering the use of the vaccine. As such, the government has said that anyone aged under 30 will be offered an alternative vaccine if one is available.

“The public’s safety is always at the forefront of our minds and we take every report of a suspected side effect very seriously indeed,” said the MHRA's chief executive Dr June Raine.

“We thoroughly analyse each and every report as we receive it and although the number of reports of CVST and other thromboembolic events has increased over the last week, so has the overall number of vaccinations administered, therefore these blood clots remain extremely rare and unlikely to occur.”

A government spokesperson said: “The Oxford/AstraZeneca vaccine is safe, effective and has already saved thousands of lives. As the MHRA – the UK’s independent regulator – and the JCVI have said, the benefits of the vaccine far outweigh the risks for the vast majority of adults.”

Ruth Rankine, director of the NHS Confederation’s PCN Network, commented: “The key message to take away from both the EU and the UK regulators is that the benefits of the AstraZeneca vaccine still outweigh any potential risks and we hope the public will be reassured that a thorough review has been undertaken and the necessary precautions taken. As Prof Van Tam pointed out, vaccination is one of our strongest tools in the fight against COVID-19, and it is vital that the programme continues at pace.”

Professor Saad Shakir, director of the DSRU, said the UK recommendation that under 30s receive an alternative vaccine “is a welcome and suitably cautious move.

“I appreciate this news will cause some people concern. I would reiterate that these events are very rare and I hope people can take some comfort from the knowledge that robust, meticulous research and monitoring is on-going into any possible side effects.