Vaccines are incredible, life-saving innovations, and COVID-19 vaccines are no different. As with any medical procedure, there are potential disadvantages to using them, but we know from extensive scientific research that COVID-19 vaccines are safe and effective.
And yet, as you probably have heard in the news, for a specific vaccine, most of Europe has stopped its launch. Instead of giving millions of people the AstraZeneca / Oxford (AZ) vaccine against COVID-19, a dozen countries have decided to stop vaccinating due to fear of side effects.
So, I thought about explaining what’s going on and why you probably don’t have to worry, even if you’ve had the AZ vaccine.
How it all works
The basic idea of launching vaccines is simple. You exclude common and dangerous side effects in clinical trials and identify all common and moderate side effects, as well as headaches and at the vaccine site.
However, even with gigantic studies, with 10,000 people in them, it is impossible to exclude things that really rarely happen. If a side effect only happens to one in 200,000 people who get the vaccine, you need a clinical trial including literally millions of people to make sure you get it in the numbers.
This raises an issue – we give vaccines at the population level, after all. Something that happens to one in 200,000 people may seem incredibly rare, and it is, but it will also happen dozens of times if you vaccinate 80 million people. And remember, it is vitally important for most of Europe that they vaccinate as many people as quickly as possible, due to the vast outbreaks of COVID-19 across the continent.
So we do something smart. We have established monitoring systems that observe people in the general population who have received the vaccine. So we can see if there are any signs of risk in the huge group of people who get the vaccine after being licensed to the general public.
What happened with the AZ vaccine is that some countries have observed a very small, but potentially significant, risk of a rare type of blood clot in their monitoring data and therefore have temporarily paused their vaccine launches to investigate it further. .
Is this a good decision? Well, it depends on a few things.
Pausing releases
The first question is quite obvious – what is the risk signal that caused the concern in Germany, Norway, Spain and elsewhere?
According to regulatory agencies in the countries involved, it is a rare but serious type of blood clot in the brain called venous sinus thrombosis. These have mainly been in very young people (20-55 years), which is part of the reason for concern.
The German public health agency published its findings, showing that there were a total of seven cases of these blood clots in the vaccinated population of 1.6 million people aged 20 to 55, when they would expect just one by chance.
Now for the real question. What this means?
Well, it is probably useful to calculate the rate here. If it is true that all seven clots were caused by the vaccine, with 1.6 million immunizations, then this results in a rate of approximately one clot per 230,000 injections or 0.00044 percent.
While the increase in relative risk here seems daunting, it means that this amounts to an absolute 0.00038 percent increase in risk for people who have been vaccinated, which is not as big as the headlines suggest.
We can also compare that to the COVID-19 risk. Even 20-year-olds are not immune to the disease and, although the risk of death is much lower than that of the elderly, it reaches about 1 death from 16,000 infections.
So if the vaccine is actually causing these blood clots, which can be fatal, the risk of dying from COVID-19 for a 20-year-old is about 15 times greater than the risk of having a clot. Again, as the two risks are very small, the absolute difference is also very small, at about 0.004%.
Observe the ‘if’. This is an open question. Remember that adverse event reporting systems exist only to identify the risk signal, not if one thing definitely causes another. This is what the ongoing investigation must do.
In addition, it is highly unlikely that the vaccine is causing these blood clots. Why? Well, Germany’s data comes from 1.6 million people, which seems a lot, but they are not the only country to give the AZ vaccine. The UK has administered more than 10 million doses of AZ and the reporting system does not show this increased risk.
If you search the MHRA reports – the UK’s medical regulator – you’ll see that there have been three cases of this specific type of post-vaccine clot in people in the UK, which is actually less than you’d expect by chance. If you combine the numbers from the UK and Germany, there will suddenly be no risk of blood clots!
Bottom line
What does all this mean for you, the person who is getting the vaccine?
Well, first of all, these risks are miniscule on an individual level. Even if this association is true – which is perfectly possible – you are at greater risk of drowning in a bathtub or being struck by lightning than having a blood clot induced by the AZ vaccine, based on the evidence so far.
Yes, the risk of blood clots can be important on a population scale, but this is very different for ordinary people like you and me.
On the other hand, the risk of death from COVID-19, even for a healthy 20-year-old, is not negligible. If you are 50 years old, the risk is very worrying. We have shown this several times, including in my own research – COVID-19 is a very unpleasant disease.
The decision to halt the launch of the AZ vaccine in European countries, of course, is up to them, but it is a little confusing. Although there may be some sign of risk, this sign is very small and is certainly no worse than catching the disease itself.
What would I do if I was offered the AZ vaccine? I can’t speak for other people, and obviously risk and benefit is something we all have to think for ourselves, but as a hospital employee in Australia, I am scheduled to receive the AZ vaccine in the next two months.
I’m looking forward to it.
Gideon Meyerowitz-Katz is an epidemiologist who works with chronic diseases in Sydney, Australia. He writes a regular health blog covering scientific communication, public health and what this new study you read about really means.
The opinions expressed in this article do not necessarily reflect the opinions of the ScienceAlert editorial team.