Covid-19 vaccines not linked to heart attack or stroke risk

The rollout of the Covid-19 vaccines saved millions of lives by turning what would otherwise be a lethal infection for some into a trivial infection for the vast majority. But there were also some concerns around potential side effects. The Astra Zeneca vaccine was linked to brain blood clots, and some speculated that the jabs might raise heart attack and stroke risks. One of the problems with studying these sorts of associations is that if the outcomes are rare, they can be hard to detect. But if you do a big study, because you have access to a literally huge population dataset, the statistical power skyrockets, and even very small differences can be detected. And that’s what Cambridge’s Sam Ip has done using “big data” collected across the pandemic by the NHS. Her findings concerning common cardiovascular complaints like heart attacks and strokes, are very reassuring…

Sam - We were very lucky to get our hands on a massive data set with 46 million adults, in fact, in England, provided by NHS England, bringing together your GP records, hospital records, Covid test and vaccination records and death data. And this allows us to study all the rare complications, common complications,

Chris - Anonymous data? All provided to you anonymously.

Sam - Yes, absolutely. They are all de-identified, provided in a secure data research environment.

Chris - Very powerful though, when you can get data at that sort of scale because presumably it helps to iron out the effects of sort of noise and the biases that creep into very small studies. You can really see the signals as they really are at a population level.

Sam - Absolutely. Firstly, it's really representative. It covers basically everyone in England and it gives us immense statistical power to study even rare conditions.

Chris - How did you actually analyse the records then? Once this comes to you, how did you then pick through it to work out what those associations were or weren't?

Sam - We looked at the data. We found out, for example, when they had their vaccinations, whether and when they had their heart problems as well as identified various other factors that could affect these associations and give you wrong answers. And we then put them all in a model that then gives us some sort of association.

Chris - And what trend emerges? Is there a link between being vaccinated or not and these health outcomes you're interested in?

Sam - Yes, there are. The main result actually is quite reassuring, which is that for common conditions, for example heart attacks and strokes and so on, the rate of these happening is actually similar or lower after people got their Covid vaccinations. So that's actually really reassuring and we, as for the rare complications that were previously reported and regulators and medical professionals are looking out for, those our study also corroborated these findings.

Chris - So that would be things like the claim that the AstraZeneca vaccine was linked to an increased risk of blood clots in the brain, for example, in certain groups. That, you saw that.

Sam - Yes, yes. But they are very transient. These effects only lasted for a very short period of time.

Chris - The fact that the rate of heart attacks and strokes and so on was lower or the same in people when they were vaccinated. To what extent could it be that the vaccine was preventing them catching Covid? Because one of the suggestions was that you catch Covid and it makes your blood stickier, it makes you have a higher risk of these sorts of outcomes. Could that be that mechanism?

Sam - This is a brilliant hypothesis and it is something that we also hypothesise that it might be true, but this is not the focus of this particular study. We're just trying to establish the links and associations between vaccinations and heart problems. But yes, this is very plausible and it's definitely worth looking at more.