What is the rare blood disease possibly related to the AstraZeneca vaccine?

The Australian federal government has called on the country’s medical and vaccine regulators to urgently review the European Medicines Agency’s discovery of a possible link between the Oxford / AstraZeneca Covid vaccine and rare blood clots.

This follows reports in recent weeks of blood clots in a small number of people around the world who had received the AstraZeneca vaccine, including a man hospitalized in Melbourne.

Scientists called the disease “vaccine-induced prothrombotic immune thrombocytopenia”. But what does this actually mean, how big is the risk, and what are the implications for the rollout of the vaccine in Australia – which currently relies primarily on AstraZeneca?

Shortage of platelets

As the name suggests, the condition is a form of something called thrombocytopenia.

Thrombocytopenia is a condition in which the number of thrombocytes (very small blood particles or platelets) is markedly reduced. Platelets form clots to stop the bleeding, so when you don’t have enough platelets in your blood, your body cannot clot. This can lead to excessive bleeding.

The disease has a genetic component, but can also result from more than 300 common drugs, including penicillin and some pain relievers. Quinine, which is added to tonic water for flavor, can also very rarely cause thrombocytopenia.

Symptoms of the disease can include severe headache, abdominal pain, seizures, and visual changes. These are similar to the symptoms of thrombocytopenia unrelated to the vaccine.

In rare cases of thrombocytopenia, clots can develop in the vessels draining blood from the brain. The European Medicines Agency said it had received reports of 169 cases of brain blood clots in people who had been vaccinated with the AstraZeneca vaccine.

In severe cases, thrombocytopenia can be fatal. There have been deaths from blood clots associated with the AstraZeneca vaccine, including 19 in the UK.

The condition appears to present four to twenty days after vaccination, and so far the problem has been largely associated with women under the age of 65.

So how could this vaccine potentially cause thrombocytopenia? The “prothrombotic immune” part of the name indicates that it is caused by overactivation of the immune system, which gives us a clue.

Platelets and Covid-19

The AstraZeneca vaccine causes cells to make a specific part of SARS-CoV-2 (the virus that causes Covid-19) called a spike protein, which the virus uses to attach to cells when it infects us.

The vaccine stimulates our immune system to generate antibodies against the spike protein, which then triggers the body to mount an immune response against SARS-CoV-2, if it encounters the virus in the future.

But in some people, the AstraZeneca vaccine seems to produce antibodies that react with the platelets, causing them to stick together, causing the blood to clot. This in turn reduces the number of circulating platelets, and therefore thrombocytopenia.

These antibodies are similar to those found in some people on a blood thinner called heparin. The immune response to heparin generates antibodies that bind to platelets. This can lead to blood clots in some people, called heparin-induced thrombocytopenia. Up to one in 20 patients receiving heparin develop thrombocytopenia.

Keeping in mind that we have not yet established a cause and effect, it is possible that the biological mechanism by which we believe heparin leads to thrombocytopenia is the same biological mechanism as the AstraZeneca vaccine.

Whether we can establish a causal link between the AstraZeneca vaccine and thrombocytopenia is under continuous investigation. Photo credit: Kai Pfaffenbach / Reuters

How common is it?

Of course, the occurring thrombocytopenia affects about one in 30,000 adults per year in the United States.

As for the suspected vaccine-induced type, according to data gathered by the Thrombosis and Haemostasis Society of Australia and New Zealand, the disease is as rare as one in 5.00,000 people. But the company notes that the data is incomplete.

Different countries have reported different rates. Norway, for example, has so far reported that one in 25,000 vaccinated adults under the age of 65 has experienced a low platelet count, bleeding and generalized thrombosis (blood clots).

Of course, the possibility that some of these cases of thrombocytopenia have occurred regardless of the vaccine makes understanding vaccine-induced cases more complicated. But overall, thrombocytopenia appears to be more common in the general population than among those who have been vaccinated.

As we continue to immunize the world, it is likely that small subgroups of people will continue to suffer from this complication. Whether we can establish a causal link between the AstraZeneca vaccine and thrombocytopenia is under continuous investigation.

Be aware, not alarmed

Amid this ongoing investigation, some countries, such as Norway, have suspended their AstraZeneca vaccine deployments. Others have restricted the use of the vaccine in certain groups, such as Canada, which only uses it for adults over 55, who may be at higher risk of Covid and lower risk of blood clots. . Meanwhile, the UK has pledged to make other vaccine options available to young people.

We will wait to see how Australian experts react. But for the general adult population, we agree with current guidelines from bodies such as the European Medicines Agency and the World Health Organization that the benefits of the AstraZeneca vaccine outweigh the risks.

Having said that, it is not unreasonable to be careful. You should watch for these symptoms for up to 28 days after receiving the vaccine:

  • shortness of breath
  • chest or stomach pain
  • swelling or cold in the leg
  • severe or worsening headaches
  • Blurred vision
  • persistent bleeding
  • multiple small bruises, reddish or purplish spots, or blood blisters under the skin.

If you experience any of these symptoms and are worried, see a doctor.

Anthony Zulli is Associate Professor and Maja Husaric is Senior Lecturer, MD at Victoria University.Maximilian de Courten is Professor of Global Public Health and Director of the Mitchell Institute and Vasso Apostolopoulos is Professor of Immunology and Associate Dean, Research Partnerships at the same institute.

This article first appeared on The Conversation.