COVID AstraZeneca vaccine UK guidance changed

Due to the potential risk of rare blood clots, under 30s are to be given the option of a different vaccine
13 April 2021

Interview with 

Michael Head, University of Southampton


A needle and bottle of the COVID-19 vaccine.


In a press conference earlier this week, the UK’s medicine regulator, the MHRA, announced a change in policy around the use of the AstraZeneca Covid-19 vaccine. This “course change”, as deputy chief medical officer Jonathan van Tam dubbed it, is because a small number of people have presented with a rare form of blood clot in the brain - called a cerebral venous sinus thrombosis - together with low blood platelets in the immediate aftermath of their first dose of the AstraZeneca agent. Now it’s not clear whether the vaccine is actually causing this, because the number of cases of the complication is so small and it’s very similar to the number of cerebral venous sinus thrombosis events that we would expect to occur by chance anyway. If we assume there is a link, among older people, the benefits of the vaccine still remain overwhelmingly clear; but because Covid-19 is a mild illness in younger people, the case for them is less compelling, and that’s why the MHRA, and their counterparts in other countries, have decided to act as they have. Chris Smith spoke to Michael Head - an infectious diseases epidemiologist at the University of Southampton who has been following the story…

Michael - So we've had a small policy change, really, in terms of the use of the AstraZeneca vaccine here in the UK. It's now not going to be recommended as a first choice vaccine for people aged under 30 - ideally we'd use one of the other vaccines, so in the UK that's Pfizer and Moderna, instead. That's due to these recent reports of thrombotic events, these rare blood clots, linked to the AstraZeneca vaccine and the most recent investigations suggest that there's probably a causal event there although there's still a lot of uncertainty around that.

Chris - Indeed they do say that, if you're in the age group for which they are recommending a different vaccine going forward, if you've already nevertheless had one dose and you're in that age group, of AstraZeneca's vaccine, you should carry on and have the second, which to me is a strong endorsement actually they think things are pretty safe.

Michael - Absolutely. And I think they have also said that the vast majority, if not all, of these thrombotic events were observed after the first dose. The regulators have expressed confidence in the vaccine itself and I think the fact that we are seeing this open, transparent decision-making around issues of safety shows that it is taken very seriously,

Chris - Can we look at the numbers then, and maybe you could use the numbers to explain why they've made the decision that they have? In other words, they've said, we're going to have one particular course of action for people of a certain age group, but business as usual for everybody else.

Michael - So they were looking at 79 cases of these thrombotic events that occurred after the first dose of the AstraZeneca vaccine. And of these cases 51 were women, 28 were men, aged between 18 and 79. And it appears to be slightly more common in younger ages within that. And the MHRA today said that it's a 4 in 1 million possible adverse event. So it is a rare event, even if they were all linked to the vaccine.

Chris - How does this relate to the background rate at which these sorts of clots occur anyway? Because this is not a new phenomenon that's only just surfaced when we started using AstraZeneca's vaccine. Cerebral venous sinus thrombosis has been happening for many years.

Michael - Yes. It is kind of tricky to work out the background rate because it appears to be quite a hard condition to diagnose accurately. The background risk appears to be around 1 or 2 per 1 hundred thousand people. And I think some of the rates that we've seen in the UK and in Europe have been a little bit elevated above that in terms of the events per number of vaccine doses. So that's partly what generated some of the safety signals that caused these investigations. So the rates appear to be a little bit higher in people that were using the vaccine, but not overwhelmingly so. So there is still that uncertainty as to whether some or all of these reported events have been linked to the vaccine.

Chris - Are we comfortable that the alternative vaccines don't have this risk? Or is it just that people haven't looked hard enough?

Michael - Well, certainly the regulators have been looking and certainly in the UK and across, I think, much of Europe, most of the vaccine doses handed out over the last few weeks have been AstraZeneca as opposed to Pfizer. But it doesn't look like there have been significant numbers of these thrombotic events in Pfizer or Moderna vaccines.


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