A small number of people have developed blood clots after receiving two types of the Covid-19 vaccine. Mirjam Guesgen explains what’s going on and how different countries are responding.

What’s this I hear about blood clots and Covid-19 vaccines?

Some people who received the AstraZeneca and Janssen vaccines are developing severe, in some cases fatal, blood clots. 

The clots have been called unusual because they crop up in different areas of the body (brain, abdomen and arteries) and people who get them often also have low platelet counts. Platelets are tiny blood cells that help stop clots from bleeding. 

The syndrome has been dubbed vaccine-induced immune thrombotic thrombocytopenia or VITT.

Countries across Europe and the UK, US and Australia and have reported cases of people developing clots after vaccination. 

How rare are we talking here?

Despite all the news buzz about them, cases of the clots are extremely rare. 

Here’s how many reports of the clots we’re currently seeing:

AstraZeneca – around three in every million people

Janssen – around one in a million people

Pfizer – no signal of clotting

Moderna – no signal of clotting

Even the highest likelihoods are still less likely than someone’s risk of developing a blood clot from taking certain oral contraceptives.

They’re also lower than the chances of being struck by lightning in the US in a year, one in 1.2 million, or in your entire lifetime in the US, 1 in 15,300. 

And they’re much lower than developing blood clots from Covid-19 itself. Based on data from the Netherlands and France, around 30–70% of Covid-19 patients in intensive care developed blood clots. 

So are the vaccines still worth it?

Yes. Health experts worldwide are saying the benefits of getting vaccinated far outweigh the risks. 

As well as greatly lowering the risk of developing severe Covid-19, there are other benefits to particular vaccines. For example, the Janssen vaccine can be given as just one dose and doesn’t have to be stored in super-cold freezers, making it really useful for vaccinating hard-to-reach people such as those who are homeless or housebound. 

Plus the Pfizer vaccine currently being rolled out across New Zealand hasn’t been linked to the clots.  

Who’s more likely to develop unusual clots?

There’s evidence that younger people – under 60 years old – and women are more likely to develop clotting. The higher number in women may be just because more women have been vaccinated, according to University of Auckland vaccinologist Helen Petousis-Harris.

Researchers haven’t been able to link the clots with any underlying medical condition.

What causes the clots?

Scientists’ best understanding right now of what’s going on is that the AstraZeneca vaccine triggers an immune response in the body.

They believe some people create antibodies against a protein called platelet factor 4, or PF4. That might lead to the platelets being pulled out of the bloodstream or destroyed, lowering platelet count. At the same time, a clot-promoting material is released, triggering clumps to form.

Researchers think this is the case because people who have developed the clots have tested positive for these antibodies and the clots are similar to a syndrome some people develop after taking the anticoagulant medication heparin. 

But scientists aren’t sure what’s setting off the immune response to the vaccines.

One theory is that it might be the vaccine carrier. A previous study in mice showed that genetic material can bind to PF4 and trigger the body to make antibodies. That might be why we’re seeing blood clotting with vaccines that use DNA as a carrier, like the AstraZeneca one. 

These viral vector vaccines have the code for making the coronavirus spike protein embedded in the DNA of a chimpanzee or human cold virus.

In contrast, the Pfizer and Moderna vaccines use mRNA, which might be why there haven’t been any cases yet of clots.

What’s being done about the clots?

Several places, including Germany and provinces in Canada, have stopped administering the AstraZeneca vaccine for people under the age of 60, opting to use the mRNA vaccines for this age group instead. Others, like Norway, have paused using the vaccine temporarily until they know more. Denmark has suspended the use of the AstraZeneca vaccine altogether. 

Several national and international public health agencies, such as the European Medicines Agency (EMA) and the US’s Food and Drug Administration, are investigating the Janssen vaccine. EMA is due to release its conclusion about the vaccine on April 20.

This includes New Zealand’s medicine regulator, MedSafe, who is asking the manufacturer for more information ahead of approving the vaccine.

Is that justified?

It really depends on where you live. 

In Germany, for example, where Covid-19 cases have been steadily rising again since February, some experts think vaccinations shouldn’t be slowed or halted. Those people include the first researcher to report on the clotting, University of Greifswald’s Andreas Greinacher. 

“People being severely sickened by Covid-19 outnumber those who suffer from the vaccination reaction by several orders of magnitude,” he said. “To stop or to avoid vaccination only for the fear of getting an extremely rare, adverse reaction would be completely wrong.”

In New Zealand, where cases are currently limited to the border and we have enough doses to vaccinate the entire population, it may make sense to take a measured approach, says vaccinologist Helen Petousis-Harris. “There’s not a rush to roll out this particular vaccine. You want to know what’s going on,” she told The Spinoff.