Many covid-19 vaccines appear to reduce the risk of heart attacks, strokes and other blood clot conditions for at least six months, according to a study of the health of about 46 million people amid the coronavirus pandemic. This is despite them causing rare side effects that affect the heart and blood clotting system.
The net benefit to heart conditions most probably happens because the vaccines protect against severe covid-19, which itself can cause heart attacks, strokes and blood clots, says at the University of Edinburgh in the UK. 鈥淲e know that covid-19 increases your risk of having these conditions.鈥�
The finding comes from the largest study of vaccine safety to date, which tracked the health of nearly all adults in England over the first year of its vaccination programme, beginning in December 2020.
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As the vaccines were rolled out worldwide, it emerged that those made by AstraZeneca and Johnson & Johnson very rarely caused a distinctive kind of blood clot syndrome, called vaccine-induced thrombotic thrombocytopenia, or VITT. In the UK,聽for example, , about 50 of whom died, out of nearly 31 million AstraZeneca jabs administered.
As a result, most high-income countries switched to the other main kind of vaccines, made by Pfizer/BioNTech and Moderna, which are based on mRNA, a strand of genetic material that makes human cells temporarily produce the virus spike protein.
These vaccines, however, were then found to occasionally cause聽inflammation of heart muscle, known as myocarditis. This is roughly as rare as VITT, happening after about 10 in every 1 million vaccinations, but it is a milder condition, usually just causing chest pain and breathlessness. Myocarditis can also be caused by infections, including covid-19.
To find out how these vaccines affected people鈥檚 cardiovascular health overall amid the pandemic, Whitely and his team took advantage of UK legislation passed early in the coronavirus outbreak that let covid-19 researchers see the population鈥檚 anonymised health records, apart from those of anyone who opted out.
This let them measure the rates of the vaccines鈥� known side effects as well as all other kinds of heart and blood-clot-related conditions that happened after getting the jabs, including heart attacks, strokes, deep vein thrombosis and blood clots in the lungs. This was compared with the rate of these events in people before they got vaccinated or who never got vaccinated, during the first year of vaccination in the UK.
While they observed the expected rise in VITT and myocarditis after people鈥檚 first vaccines, they also saw about a 20 per cent lower rate of more common conditions, such as heart attacks and strokes, once the figures had been adjusted to take account of the fact that older and sicker people were more likely to be vaccinated. The lowering of these common conditions far outweighed the incidence of the very rare side effects, leading to a net benefit.
at the Open University in the UK says the conclusions hinge on whether the figures were adjusted well enough for possible biases, such as the fact that people may have postponed vaccination if they were in poor health, which would have made the vaccine results look better than they were. 鈥淗owever, this is reassuring in that we didn鈥檛 find any evidence that [vaccination] made cardiovascular risk higher,鈥� he says.
鈥淭his is the best data that we have so far and it鈥檚 very reassuring,鈥� says at Thrombosis UK, a charity for people affected by blood clots.
medRxiv
