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A growing number of people are suffering from blood clotting disorders after their second dose of AstraZeneca’s coronavirus vaccine, data shows.
The UK’s medical regulator found 15 cases in people recently given a top-up dose by May 12, the most recent count, up from six at the start of the month.
So far 9million Britons have been given two doses of AstraZeneca’s jab, meaning the extremely rare clots are occurring in around one in 600,000 people.
Scientists told MailOnline it was ‘disappointing’ the extremely rare complication was becoming more frequent in double-jabbed patients. The clots — which can occur in the brain — are happening alongside abnormally low platelet levels, known as thrombocytopenia.
But the Medicines and Healthcare products Regulatory Agency said symptoms were ‘milder’ and less frequent than after the first dose.
As of May 12, the MHRA had spotted 294 cases of the clots in Britons given an initial injection, affecting about one in 80,000. The conditions were found to be occurring more frequently in young people, which has led to the British jab being restricted for use in under-40s.
Scientists believe in some people, the immune system sees the vaccine as a threat and over-produces antibodies to fight it. These lead to the formation of clumps in the bloodstream, which can become deadly if the clots move towards vital organs and cut off supply.
Fifteen people have suffered from blood clots after getting their second AstraZeneca vaccine (file)
Around 15million people are still waiting on their second AstraZeneca vaccine, with millions of eligible under-40s yet to be fully inoculated.
The current guidance says younger people who had their first dose before the jab was restricted should come forward for their second.
Department of Health bosses do not provide data on vaccine take-up by age, making it impossible to say how many younger adults are booked in for their top-up.
Professor Paul Hunter, an expert in medicine at the University of East Anglia, said it was ‘disappointing’ there had been more clotting cases after the second dose.
But he highlighted that they were even less common than a first dose and less severe, urging people to continue to come for their second.
He told MailOnline: ‘It is disappointing. There was some hope that this may have just been a first dose issue.
‘If 15 of the 309 cases so far reported are following second dose, then the risk after second dose does seem lower, even after accounting for the fewer second doses that have been given.
‘So the advice remains to present for your second dose when called.’
Dr Simon Clarke, a microbiologist at Reading University, told MailOnline that there could be an uptick in second dose-related clotting disorders now that the vaccination programme is deploying more second than first doses.
‘It’s entirely possible that there will be more clotting episodes as the number of second doses increases,’ Dr Clarke added.
‘Younger people seem to be at higher risk of these blood clots, but they are relatively unlikely to have had their second dose quite yet – that will change soon, so we’ll see as we move through the second doses.’
The MHRA said clotting after second AstraZeneca doses had risen ‘in line with the number of second doses given’.
A spokesperson added: ‘Blood clots with lowered platelets occur extremely rarely and the incident rate after a second dose remains extremely low, at around 1.7 cases per million second doses given.
‘Our advice remains that you should continue to get your first and second dose of COVID-19 Vaccine AstraZeneca unless specifically advised not to do so. We continue to keep all cases of these rare blood clots under close review.’
The UK Government opted to make it policy to give adults aged 40 or younger a choice of vaccines when they go to their appointment because the rate of the clots with low platelets appeared more common among them, at around one in 60,000 people.
For older people who are at a genuine risk of dying if they catch Covid, the benefits of protection from the virus clearly outweighed any negative side effects.
Experts said the infection rate in the UK is so low that the risk of the rare clots outweigh that of Covid in younger adults, who often only suffer mild illness.
But the absolute risk of the clots is still tiny. Oxford University analysis has shown that the risk of suffering clotting after catching Covid is eight times higher than through vaccination.
In its most recent report, the MHRA said that of the 309 clotting cases reported so far, 169 were women and 138 men.
The patients were aged 18 to 93 years and the overall case fatality rate was 18 per cent with 56 deaths.
Cerebral venous sinus thrombosis – a dangerous clot in the brain – was reported in 116 cases, with an average age of 46.
A total of 193 had other major thromboembolic events, with an average age of 55 years, with low platelet counts.
The MHRA concluded: ‘On the basis of this ongoing review, the advice remains that the benefits of the vaccine outweigh the risks in the majority of people.’
Scientists believe the clotting conditions are being triggered by a dangerous overreaction by the immune system.
It causes the to body attack its own platelets — tiny chunks of cells inside blood that build clots to stop bleeding when someone is injured.
The immune system overproduces antibodies – normally used to fight off viruses – which mistake platelets in the blood for foreign invaders and attack them.
To compensate, the body then overproduces platelets to replace those that are being attacked, causing the blood to thicken and raising the risk of clotting. This then causes levels of platelets to fall.
Researchers say the phenomenon is similar to one that can occur in heparin-induced thrombocytopenia (HIT), when sufferers take a drug called heparin.
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