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The pandemic has now entered a phase where it is largely a battle between the virus and the various vaccines that effectively produce immunity to the virus. We have an ambitious program, with many vaccines and millions of people being vaccinated every day. As a consequence in those countries with high vaccine uptake, the infection rate is starting to decrease, although worry of new strains emerging remains.

As millions get the jab, safety is also being monitored closely. These are all new vaccines, and although they appear highly safe when tested in tens of thousands of people, less common risks can emerge when you give any vaccine to tens of millions of people. So far, the Moderna and Pfizer vaccines being used in the US have shown no concerning signs of significant side effects. The same is mostly true for the AstraZeneca vaccine being widely used in the European Union, except for recent case reports of blood clots occurring in some people who received the vaccine. This has led to a controversy over whether the vaccine is safe and whether or not to suspend its use.

Denmark has already suspended use of the AstraZeneca vaccine, followed quickly by Norway, Iceland, Germany, France, Italy, Cyprus, Spain, Latvia and Sweden. The UK continues to use the vaccine without pause. What does the current science have to say about this controversy?

AstraZeneca reports that: “across the EU and the United Kingdom there had been 15 events of deep-vein thrombosis (DVT) – a blood clot in a vein – and 22 events of pulmonary embolism.”

Of course, the big question is – is this causation or mere correlation? With millions of people getting vaccinated, all of the usual life events are going to continue to occur, and some people will get sick, have events, and even die soon after getting the vaccine just by chance alone, without any causal link to the vaccine. This is the background noise that experts monitoring the safety of the vaccines, or any medical intervention, have to deal with.

The way to tell if there is a causal link to the vaccine is to compare the observed rate of events with the expected background rate, and make a statistical comparison. A probability of a link can then be calculated. The World Health Organization (WHO) has done this, and concluded that there is no evidence of a link between these cases and the vaccine. The UK’s Medicines and Healthcare products Regulatory Agency has come to the same conclusion.

To give an idea of why, DVTs alone occur in about 80 people per 100,000 per year. If, say, we consider the background rate in 10 million people that’s 8,000 per year, or 307 in any two week period. The numbers being reported for various kinds of blood clots are still at these background rates, or even lower – especially given that those getting the vaccines first are in high risk groups who have a higher background rate of blood clots than the general population. In other words, there is no signal peaking up above the noise.

So far we are only talking about risk (which is low to none) but we can also take another approach to this – risk vs benefit. We could ask, for example, even in the worst case scenario that every single reported blood clot after receiving the AstraZeneca vaccine were caused by the vaccine, would that risk be greater than the benefits of getting the vaccine (or the risks of not getting the vaccine)? Interestingly, COVID itself can rarely cause blood clots, so we can directly compare the total risk of getting blood clots from COVID and the vaccine in vaccinated vs unvaccinated people. There, too, the risk of the vaccine is less than the risk of COVID – less than the risk of not getting vaccinated.

And keep in mind – that is the worst case scenario, which is almost certainly not true. It would mean that none of the reported blood clots were background events, which is statistically highly unlikely.

Unfortunately, people are risk averse, and we are more afraid of directly causing harm than allowing harm to occur by not taking action, even if the end result is just as bad. This is why public messaging on this question has to be clear. Scientists, however, communicate in a technical style that is more precise, but can be confusing to the public, even making it sound like there is a risk when there isn’t.

The countries who have suspended use of the vaccine have been highly criticized for their decision, given that it is not supported by the science. These background events are going to crop up when millions of people are being given a medical intervention and being closely monitored. Suspending the vaccine seems like an overreaction, given the data, with likely real-world consequences. These decisions have likely caused more deaths than they hoped to prevent. That is the reality of a deadly pandemic – public health decisions can be critical, and mistakes in judgement have a death toll attached to them.

The experts are all in agreement here – the reported events are likely background events not related to the vaccine, and in any case the benefits of getting vaccinated still vastly outweigh any potential risks. The countries who have suspended the vaccine should reverse course and resume vaccinating their people.

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Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.