Deciding to take up a vaccination is a “balance of risk and benefit”, according to experts – but the advantages of the AstraZeneca jab still “far outweigh” any of its risks .
Receiving the AstraZeneca vaccine affords protection against becoming severely ill from Covid and is also likely to reduce transmission.
But the average risk for an adult is around a one in 250,000 chance of developing a very rare blood clot, according to Dr David Spiegelhalter, a statistician from the University of Cambridge.
For older adults the risk is even less – around one in half a million. For people in their 20s, it is around one in 100,000, Dr Spiegelhalter said.
In a joint press conference with the joint committee on vaccination and immunisation (JCVI) on Wednesday, the Medicines and Healthcare products Regulatory Agency (MHRA), iconceded there is a possibility that the jab is causing rare brain blood clots .
In light of this, the JCVI has recommended that people under 30 should be offered alternative vaccines.
But what is the risk, and how does it compare to others we take in daily life?
“If you vaccinated Wembley Stadium, full up with people in their 20s, we would certainly expect one to get one of these effects,” Dr Spiegelhalter told BBC Radio 4's Today programme.
“However, not only [do vaccines] prevent hospitalisations and some intensive care admissions in the others, but think of the prevention of transmission that we would get by vaccinating that many young people.”
The risk of dying in a road accident over the course of three months is also one in 100,000 for someone in their 20s, Dr Spiegelhalter added, or in “any sort of accident over the month”. “It is a very rare risk,” he said.
Statisticians often measure risk using “micromorts” – one micromort represents a one in a million chance of dying. The measurement has been used frequently throughout the pandemic to explain the risk of dying from the virus and can be translated into odds.
When using micromorts to understand the risk of dying from other activities and behaviours, the risk of developing a rare blood clot from the AstraZeneca jab pales in comparison.
The risk of dying from a rare blood clot after receiving the AstraZeneca jab is around one in 1,000,000, the data released by the MHRA so far suggests.
The risk of dying when giving birth is 80 micromorts, around one in 12,500. After receiving a general anaesthetic your risk is 10 micromorts – a one in 100,000 risk.
The chances of being murdered in the next year are also around one in 100,000, while dying in a hang-gliding accident is around one in 125,000.
However, the risk of an adverse effect from a vaccination is different to something that could happen in the course of everyday life, Dr Spiegelhalter said.
That is why the “cautious” decision has been made to minimise the potential harm, he explained.
Prof Jonathan Van Tam, the deputy chief medical officer, told Wednesday’s press conference that this is a “vanishingly rare, but sadly quite serious adverse effect”.
“This is a course change, it is based on a clinical preference, based on newly emerging data,” he added.
Prof Anthony Harnden, the deputy chairman of the JCVI, said: “What is clear is that for the vast majority of people the benefits of the Oxford/AZ vaccine far outweigh any extremely small risk and the Oxford/AZ vaccine will continue to save many from suffering the devastating effects that can result from a Covid infection.”
Matt Hancock, the Health Secretary, told BBC Breakfast: “The safety system that we have around this vaccine is so sensitive that it can pick up events that are four in a million. I’m told this is about the equivalent risk of taking a long-haul flight.”
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