You have a higher risk of getting blood clots from contracting Covid-19 than from getting the first dose of either the Pfizer or AstraZeneca vaccine, new research shows.
In what is believed to be the largest study of vaccine side effects to date, University of Oxford researchers analysed the health data of more than 29 million people aged 16 and over who received their first doses of either vaccination. The data was collected between December 2020 and April 2021
Researchers used routinely collected electronic health records to analyse the short-term risks (within 28 days) of hospital admission from blood clots, looking for any complications after a patient was either infected or vaccinated.
The study’s findings suggest the risk of thrombocytopenia, a condition where the patient has a low count of cells, known as platelets, that help the blood clot, in someone with coronavirus is almost nine times higher than in someone who has had one dose of the AstraZeneca jab.
These findings come a day after an inquest heard that an award-winning BBC radio presenter died due to complications of the AstraZeneca vaccination.
BBC Radio Newscaster presenter Lisa Shaw died in May, aged 44, just over three weeks after she had her first dose. Shaw developed headaches a week after the jab and then suffered blood clots that led to a brain haemorrhage
The coroner said on Thursday that Shaw had developed a vaccine-induced thrombosis and thrombocytopenia, described as “a rare and aggressive complication associated with the AstraZeneca vaccine” and ruled to be “the underlying cause of her death”.
Here’s what experts want you to know about the risk of blood clots from coronavirus and Covid vaccines in light of the new research.
Serious vaccine side effects are very rare
Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford and lead author on the new vaccine study, said that Shaw’s death was “very sad”, but that any potential risks of the vaccine compared with Covid-19 infection must be put into context.
“This is very sad and condolences to the family of Lisa,” she told BBC Radio 4’s Today programme. “But to put it in context, these are very rare cases, and the vast majority of patients will be absolutely fine with these vaccines.”
An expert, who was not involved in the research but who is a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M), which advises the government, said there will be always be some “tragic cases”, but it is “so much more risky to catch Covid”.
Dr Mike Tildesley said he hoped the new study, which involved millions of people, “maintains the trust in the vaccines going forward”.
“There are always going to be the tragic cases like Lisa unfortunately, but it still doesn’t mean that actually the risks of taking the vaccine are high, it is still so much more risky to catch Covid and develop a blood clot via that route,” he told BBC Breakfast.
Here’s how the risk of blood clots compares
Professor Aziz Sheikh, co-author on the study, said the increased risk of thrombocytopenia seen in their work is similar to other commonly used vaccines in the UK, such as the flu jab.
Researchers estimated that in 10 million people vaccinated with the AstraZeneca jab, there would be 107 additional cases of thrombocytopenia in the 28 days post-vaccination, compared with 934 in people with the virus.
The scientists found an association between those vaccinated with the Pfizer jab and an increased risk of stroke, but the risk was more than 10 times greater in those with the virus: an estimated 143 extra cases per 10 million people, compared with 1,699 cases in those with Covid-19.
The paper also said that per 10 million people jabbed with AstraZeneca, there were an estimated seven additional cases of CVST, while there were 20 in people with Covid-19. For blood clotting in a vein, they estimated 66 additional cases per 10 million people vaccinated with AstraZeneca, compared with around 12,614 in those who has contracted Covid-19.
There were also no associations with blood clots in an artery for either vaccine, while there were some 5,000 cases per 10 million people infected with Covid.
Prof Hippisley-Cox said the increased risks they detected were only for a short time after vaccines, compared to a longer period if infected with the virus. “For stroke, with Pfizer it was just 15 to 21 days after vaccination that there was an increased risk. And for thrombocytopenia with the AstraZeneca it was eight to 14 days,” she said.
“They were very specific, short periods of time, whereas the associations with infection appeared to be generally over a whole 28-day period after the infection.”
Most vaccine side effects are very mild
Fears over blood clots after vaccination have caused some vaccine hesitancy and have meant that the use of the Oxford-AstraZeneva vaccine in particular is restricted in a number of other countries.
According to previous research published in the Lancet medical journal in July, the Astra-Zeneca Covid-19 vaccine doesn’t raise the risk of rare blood clotting after a second dose.
More than 90 million vaccine doses have so far been administered in the UK – 47,860,628 people have got their first dose (88% of over-16s) while 42,860,628 people are fully vaccinated – more than three quarters of the adult population.
Only one in four people experience side effects after receiving either vaccine, according to analysis of data from the Zoe Covid Symptom Study app and most people report those side effects as “mild and short lived”, peaking within the first 24 hours following vaccination and usually lasting one to two days.
The vaccines have been extensively trialled
All three Covid-19 vaccines currently in use in the UK – AstraZeneca, Pfizer BioNTech, and Moderna – have been through randomised clinical trials, large enough to detect very rare adverse side effects.
Regulators perform a risk-benefit analysis of the medicine to compare the risk of the adverse reaction when rare events are uncovered such as a blood clot, against the benefits of avoiding the disease.
In fact, analysis of data from the Zoe Covid Symptom Study app reassuringly found fewer side effects in the general population with both the Pfizer and AstraZeneca vaccines than reported in clinical trials by their manufacturers.
The researchers on the new vaccine study were independent from the University of Oxford team that worked with AstraZeneca to develop its vaccine.
Vaccination is still the best way to protect yourself – and others
Vaccines are estimated to have now prevented more than 100,000 deaths, according to the latest information from Public Health England.
Prof Aziz Sheikh, co-author of the University of Oxford study said the findings “clearly underscore” the importance of getting vaccinated to reduce the risk of these clotting and bleeding outcomes. Vaccinations, he said, offer a “substantial public health benefit”.
“People should be aware of these increased risks after Covid-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2,” concluded Professor Hippisley-Cox.
This article originally appeared on HuffPost UK and has been updated.