What you need to know about AstraZeneca and blood clots

TVO.org speaks with McMaster University’s Menaka Pai about symptoms, proper care, and the risks associated with getting a second dose
By Daniel Kitts - Published on May 13, 2021
Hematologist Menaka Pai says that the science around vaccine-induced blood clots is quickly evolving. (McMaster University)



Ontario announced Tuesday that it was suspending use of AstraZeneca’s COVID-19 vaccine over concerns that it can cause potentially fatal blood clots. Health authorities have emphasized that your chance of getting a clot from the vaccine is very small and that, if you develop one, it is treatable if it’s caught quickly enough. That’s left Ontarians — especially those who’ve already received a shot — with a lot of questions.

So what are the potential symptoms of a clot, and what should you do if you think you have one? TVO.org speaks with Menaka Pai, associate professor of medicine at McMaster University and hematologist and thrombosis medicine physician at Hamilton Health Sciences, about what we know now and how the science is evolving.

TVO.org: What are the symptoms of a potential blood clot caused by the AstraZeneca vaccine?

Menaka Pai: I’m going to give you a laundry list, but the laundry list sort of corresponds to symptoms of blood clots that can be found with this syndrome, which is called VITT [vaccine-induced immune thrombotic thrombocytopenia], so let’s start from the top down.

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If you have a blood clot in your brain, you’re going to feel a severe headache that doesn’t go away. You may have seizures. You may have visual problems — blurry or double vision that’s new. You may have difficulty moving or speaking.

And then as we kind of move our way down [the body], blood clots in the lungs or body can present as shortness of breath, chest pain, back pain, or abdominal pain. And then we also talk about blood clots in the limbs. Those could involve a pain in the limbs — an arm or a leg — and often swelling or colour change. So these are all new symptoms [for the individual]. They’re severe symptoms, and they occur at a four- to 28-day window after the shot.

TVO.org: How long should these symptoms persist before you become concerned?

Pai: Well, that’s a million-dollar question, because everyone has a different risk threshold, right? What I tell people is, “You really know your own body, your own symptoms.” A headache that gets better with one Tylenol and doesn’t come back is fine. But if it’s something that’s not going away, and it’s interfering with your function over six, 12, 24 hours, that’s a reason to get help. If it’s mild, that just might mean calling your own doctor. But if it’s severe — new double vision for example — you need to get to a hospital for that.

TVO.org: If you think you might have a clot caused by the vaccine, what should you do?

Pai: I think the real key is seeking care. These clots are super-rare, but they’re serious, and they’re aggressive. What we are finding is that early intervention helps. What that means is: don’t panic. If you start feeling these unusual symptoms, you contact a medical professional. If they are mild symptoms, that really should be your own doctor, or Telehealth, or a walk-in clinic. But anything where it’s severe and nothing’s working, that’s where you go to the nearest emergency department.

TVO.org: How treatable is this type of clot?

Pai: Another loaded question. There’s a lot of focus on fatality. Early on — we only discovered this about seven or eight weeks ago — when we really didn’t know what this was or how to treat it, we were looking at fatality rates of 40 per cent. Case fatality now has come down to about 20 per cent.

We consider this treatable, in that we have treatments for it, and most people survive. One thing you might want to highlight, though — and I’m trying to highlight when I talk about this — is treatable doesn’t mean that it’s not a problem. For example, I have treatments for the strokes that are caused by VITT. But often that patient might be left with permanent disability. It’s not like I treat them and then they just walk out 100 per cent. So there’s a mortality of about 20 per cent, but the morbidity, the harm, that’s tougher to estimate. But we know that these clots cause harm, even if they don’t kill you.

TVO.org: You mentioned that time is of the essence. Is there any sense of how quickly this clot has to be diagnosed to be treated properly?

Pai: It’s so tough to generalize. And I think that’s because VITT can cause blood clots in really any artery or vein in the body. We know that the clots in the head can be aggressive. If you leave them for more than a couple of days, they may get worse quickly. And we’ve seen, for example, a very sad story out of Edmonton about a woman who sought care, [the clot] wasn’t picked up, and it got left for a couple days. Clots in the limbs, you probably have a little bit more time to get to your doctor. But we know that these clots essentially don’t improve if they’re not treated.

TVO.org: You mentioned the woman in Edmonton who died. She’s one of a few stories of people allegedly going to the hospital with symptoms, being sent home, and then suffering severe strokes or dying from an AstraZeneca-induced clot. How confident can people be that if they seek medical assistance for a potential clot that they will get the help they need?

Pai: That’s such a tough question. It’s hard for me because hindsight is 20/20. It’s so easy for us to look back on these really awful cases and say, “That doctor should have known this, or that nurse should have drawn this blood test.” But the reality is that the symptoms are a little bit vague. And for the last seven, eight weeks, we’ve been learning more, and we’ve been enhancing our awareness. My number one, 24/7 goal right now — why I’m talking to you — is just to get this on the radar of every patient and every health-care professional. I think the best way to deal with this if you have gotten AstraZeneca is to know about it, and then you feel empowered, and you advocate. And then people like me will do our best to get this on the radar of every single health-care professional.

TVO.org: Are there specific things you should ask a doctor to make sure they are properly checking for a vaccine-induced clot and they know how to treat it correctly?

Pai: There’s a couple things I do suggest to people, and they’re actually pretty straightforward. The first thing is you tell the doctor or nurse or any health-care professional: “Here’s when I got the shot.” Because we really believe that date is important — four to 28 days. So you’re helping the doctor put the puzzle together by giving them the date piece. And you tell them what kind of vaccine it was. And then I guess the other thing is, if you have these odd symptoms, we’re telling docs and nurses, “Do a complete blood count, a CBC.” A complete blood count gives you a lot of info, and one of the pieces of info it gives you is it shows you whether the platelet count is low. And that’s really important.

TVO.org: There’s a lot of discussion now about whether those who have already gotten an AstraZeneca dose will be given a second dose or whether they’ll be given something else. What do we know about blood clots among those who had the second shot?

Pai: Science evolves very quickly. We don’t have as many second doses as first doses in this world, so while it’s probably reasonable to say that risk of a clot is likely lower with the second shot than the first, we know that the risk isn’t zero. Our estimate is that it’s one in 1 million, but it’s still too early to tell if that is a precise estimate or if it’s a ballpark estimate. You only get more precise when you get more numbers. I would say it is likely lower with the second dose. But we need more information to make a really good prediction for people.

This interview has been condensed and edited for length and clarity.

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