Blood clots declared a very rare side effect of AstraZeneca COVID-19 vaccine – UK/Aus SMC Expert Reaction

Medicines regulatory bodies in the UK and Europe have confirmed a potential link between the AstraZeneca Covid-19 vaccine and blood clots and low blood platelets.

Below are excerpts of comments that the UK and Australian Science Media Centres have gathered comment in response to the statements. See the Covid Vaccine Media Hub for the latest information.

Dr Jose Perdomo, Senior Research Officer, Haematology Research Unit, St George and Sutherland Clinical School, UNSW, comments:

“It appears very likely that the AstraZeneca vaccine triggers a unique immune reaction characterised low platelet count and clotting. This is similar to another relatively rare, but serious clotting condition, caused by the use of heparin. This type of immune reaction involves platelets and white blood cells, and that is the reason for the development of severe clotting. In both cases, clotting appears within two weeks of administration of the substances.

“Clotting is seen in arteries and veins at unusual sites in the brain and abdomen and mortality is high.

“It should be emphasised that so far these are considered rare events and the benefits of the vaccine still outweigh the risks.  A cautious approach is recommended since specific risk factors are currently unknown. More research into the causes, risk factors and treatments are urgently needed.”

No conflict of interest.

Associate Professor Paul Griffin, Director of Infectious Diseases, Mater Health Services; and Associate Professor of Medicine, University of Queensland, comments:

“To keep in context, unfortunately no medical intervention is free of risk, and the risk of these clotting issues remains exceedingly rare at less than 5 per million doses administered. For comparison, a very common intervention in the form of the oral contraceptive pill carries a risk of clotting issues in the order of four of every 10 000 people taking this pill. When assessing the risks and benefits we also have to keep in mind the risks of not vaccinating.”

Conflict of interest statement: Paul is on the AstraZeneca advisory board and is also trialling other COVID vaccines including Novavax.

Professor Arne Akbar, President of the British Society for Immunology, comments:

“After careful analysis, the JCVI have issued new guidance concerning who should receive the AstraZeneca/Oxford COVID-19 vaccine. This will have been the result of dozens of experts who have many years of experience in this area carefully scrutinising the available data.  To be ultra cautious, they have advised that while the benefits of vaccination continue to outweigh the risks for everyone, people under 30 with no other health conditions should be offered an alternative vaccine to the AstraZeneca/Oxford vaccine where available.

This is due to very rare reports of individuals developing blood clots with low platelet counts after receiving this vaccine.  While the evidence is still accumulating to allow us to say definitively whether these symptoms are linked to the vaccine or not, the JCVI is taking a careful, proactive approach in acting now. The reason for this shift in emphasis for younger people is because the benefit to risk ratio is less pronounced than in older people who are at higher risk of severe COVID-19 disease. We still don’t have a clear understanding of what the mechanism behind these potential side-effects could be. Although there is some early evidence that there is an adverse immune response occurring that affects the platelets, we urgently need more research to clarify this.”

No conflict of interest declared.

Professor Ian Douglas, Professor of Pharmacoepidemiology, London School of Hygiene & Tropical Medicine, comments:

“The messages from both MHRA and EMA are in close agreement, which is reassuring.  It’s also impressive to see the vaccine safety monitoring systems throughout Europe operating at such speed, with great rigour and transparency. The evidence from the case reports seems to be compelling enough to conclude at least a possible causal link between the AZ vaccine and rare clots accompanied by low platelet counts. Though I note the caution expressed here; it is still not proven beyond doubt. Two things do stand out beyond doubt. Firstly the well quantified and large benefits provided by all the currently available vaccines, and secondly that any risk from clotting appears to be very rare. The balance of risks and benefits shown by Prof Jonathan Van Tam for different age groups and in different levels of circulating virus helped demonstrate this to some degree, but we need to remember the risks from COVID are of course greater than admission to intensive care alone. Many people suffering consequences of COVID-19 such as clots and other long term problems won’t have been in intensive care but these are of course real and harmful side effects of COVID-19 and which need to be included in our assessment of the risks and benefits of vaccines.

“The move to recommend Pfizer or Moderna vaccines for under 30’s in the UK is a very cautious approach and makes some sense. Though we need to remember this doesn’t mean other vaccines are not also associated with rare clots, just that we have less evidence and experience with them at the moment.

“The reality of the incredible COVID vaccine roll out speed and volume means we will continue to find out more about these rare clots and we may well see new safety signals with any of the vaccines. This is the nature of all effective medications; their rarer side effects only emerge when we use them at large scale.”

No conflict of interest declared.