Of the nearly 21 million people given first doses of the Oxford/AstraZeneca jab, 100 have now suffered a rare immune system-linked blood clotting syndrome, according to the latest data published by the UK medicines regulator.
The Medicines and Healthcare products Regulatory Agency (MHRA) also said that 22 of those people have died.
It advises the benefits of the vaccine at preventing Covid remain higher than any potential risk in all but the youngest age groups. Nearly 48,000 people have died of Covid since the vaccine was introduced.
The condition, where unusual blood clots occur in conjunction with low platelet levels in the blood, has now been detected in 61 women and 39 men aged from 18 to 85-years-old.
Of these, 99 cases were in people who had received their first dose of the vaccine. Half of them had a rare form of blood clot in the brain called CVST. The rest had other major blood clots elsewhere. The MHRA says the one case following a second dose of the vaccine may have been caused by other medical conditions in the individual concerned. Reassuringly, the percentage of people who have died from the syndrome appears to have decreased as a proportion of cases. Blood clotting experts attribute this to increased awareness of the risk of the side-effect and a protocol for treating it more effectively. Given the continued roll-out of the vaccine, the MHRA was expecting more cases of the condition.
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Thursday's numbers are an increase of 21 cases and three deaths on the week before when the evidence suggesting of a link between the vaccine and blood clots in the UK was first confirmed. There have been 20.6 million first doses of the AstraZeneca vaccine given leading to an overall risk of 4.9 per million doses. It's a slight increase on the figure of four per million reported last week, possibly due to increased vigilance for the condition among doctors. The new data still suggest the syndrome may be slightly more common in younger age groups, reinforcing the decision to pause the use of the vaccine in people under 30.
However, this decision was also based on the fact young people are less likely to from Covid in the first place. If infection rates were to increase and with it any one person's risk of catching Covid, the risk-benefit profile of the vaccine could tip in favour of younger people again.
The increase in cases strengthens evidence for a causal link between the vaccine and the rare side effect. It's something vaccine specialists say they need to urgently understand. Reports of six cases of blood clots with low platelet count, and one death, have been linked to 6.8 million doses of the Johnson & Johnson Covid vaccine given in the United States. The US numbers are far too small to have any statistical significance but the J&J vaccine is of a very similar design to the AstraZeneca jab. If this rare side effect is in some way linked to the vaccine's design, it could point to a potentially wider problem. Although, in many parts of the world right now, infection rates are sufficiently high. As a result, the blood clot risk associated with the vaccine is insignificant compared to the risk of dying from Covid.