The benefits associated with COVID-19 vaccine administration (both Pfizer/Comirnaty and Oxford/AstraZeneca) outweigh a potentially increased risk of blood clots with low platelet counts following Oxford/AstraZeneca vaccination among people aged under 50.

Key points

• Very rare instances of blood clots with low platelet counts (thrombosis with thrombocytopenia) have been reported among people who have received the Oxford/AstraZeneca vaccine.
• Overall, the benefits associated with COVID-19 vaccine administration (both Pfizer/Comirnaty and Oxford/AstraZeneca) outweigh a potentially increased risk of thrombosis with thrombocytopenia following Oxford/AstraZeneca vaccination among those aged under 50.
• In Australia, it has been recommended that the Pfizer/Comirnaty vaccine be preferable for all adults under 50 years of age.
• Where benefits outweigh the risks of thrombosis with thrombocytopenia among adults under 50 years of age, the Oxford/AstraZeneca vaccine may still be administered.
• People who have received the first dose of the Oxford/AstraZeneca vaccine with no serious adverse effects (including people under 50) should still receive the second dose.

Introduction

In Australia, COVID-19 vaccines are now being administered in line with the national rollout strategy. COVID-19 vaccines protect people against the ‘severe acute respiratory syndrome coronavirus 2’ virus (or ‘SARS-CoV-2’). Approved COVID-19 vaccines may be effective in reducing the severity of illness but may not completely protect against infection or prevent transmission to others. Vaccinated people should continue to follow all current official recommendations regarding infection prevention and control.

As rollouts are progressing at different rates, developers, researchers, organisations, and governments around the world continuously monitor, analyse, and report on the safety and effectiveness of COVID-19 vaccines. The Australian Therapeutic Goods Administration (TGA) has provisionally approved the Oxford/AstraZeneca viral vector vaccine and the Pfizer/Comirnaty mRNA-based vaccine for administration in Australia.

The AstraZeneca vaccine and blood clots with low platelet counts

Recently very rare instances of blood clots with low platelet counts (thrombosis with thrombocytopenia) have been reported among people who have received the Oxford/AstraZeneca vaccine.^1,2  An in-depth review of 62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis has been carried out from a total population (to date) of approximately 25 million people who have received the vaccine in Europe.¹

It is important for healthcare professionals and people who receive the vaccine to be aware of the very rare possibility of blood clots combined with low levels of blood platelets occurring within two weeks of vaccination.¹ People who have been vaccinated should seek medical assistance immediately if they develop the following symptoms:¹

• shortness of breath
• chest pain
• swelling leg
• persistent abdominal pain
• neurological symptoms, including severe and persistent headaches or blurred vision
• tiny blood spots under the skin beyond the site of injection

To date, one case of thrombosis with thrombocytopenia has been reported in Australia and has been assessed by the Vaccine Safety Investigation Group (VSIG).³ As yet, there is insufficient evidence to confirm that the clot was caused by the vaccine, however emerging evidence internationally suggests a likely association.¹ The European Medicines Agency (EMA) has found a possible link between the AstraZeneca COVID-19 vaccine and thrombosis with thrombocytopenia.¹ Most cases have been in women under 60 years of age and within two weeks of vaccination.¹ The Australian Therapeutic Goods Administration (TGA) is carefully reviewing all Australian reports of blood clots following the AstraZeneca vaccine and will update any advice accordingly.⁴

In Australia, it has been recommended that the Pfizer/Comirnaty vaccine be preferable for all adults under 50 years of age (i.e. all adults regardless of sex assigned at birth).³ Where benefits outweigh the risks of thrombosis with thrombocytopenia among adults under 50 years of age, the Oxford/AstraZeneca vaccine may still be administered.³ Overall, the benefits associated with COVID-19 vaccine administration (both Pfizer/Comirnaty and Oxford/AstraZeneca) outweigh a potentially increased risk of thrombosis with thrombocytopenia following Oxford/AstraZeneca vaccination among those aged under 50.^1,3 People who have received the first dose of the Oxford/AstraZeneca vaccine with no serious adverse effects (including people under 50) should still receive the second dose.³

Globally, many groups are continuously monitoring even very rare and minor adverse events. Even an extremely small number of cases of blood clots among vaccine recipients is being taken seriously and investigated. It is important to consider that among non-vaccinated people, blood clots are relatively common (millions of cases every year worldwide).⁴ There are many and varied risk factors that increase a person’s risk of blood clots including some common medications (e.g. oral contraceptives), hospitalisation and medical treatments (major surgery), pregnancy and postpartum, increasing age, male biological sex, and personal and familial history).⁴

That even these very rare cases have been identified and reported highlights the degree to which authorities in Australia and around the world are carefully reviewing emerging evidence about the safety and effectiveness of the COVID-19 vaccines. It is important to be aware that based on the current evidence and advice, the benefits associated with COVID-19 vaccine administration (both Pfizer/Comirnaty and Oxford/AstraZeneca) outweigh a potentially increased risk of blood clots with low platelet counts following Oxford/AstraZeneca vaccination among people aged under 50.

We are currently updating our resources to reflect this emerging evidence. For ANMF resources on the COVID-19 vaccines, please follow this link:

• ANMF COVID-19 Vaccine Resources

References

1. European Medicines Agency (EMA) AstraZeneca’s COVID-19 vaccine: EMA finds possible link to very rare cases of unusual blood clots with low blood platelets. 2021. Available from: https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-finds-possible-link-very-rare-cases-unusual-blood-clots-low-blood
2. Dyer O. Covid-19: EMA defends AstraZeneca vaccine as Germany and Canada halt rollouts BMJ. 2021; 373 :n883 doi:10.1136/bmj.n883
3. Australian Government Department of Health. Joint statement on COVID-19 AstraZeneca vaccine advice from ATAGI. 2021. Available from: https://www.health.gov.au/news/joint-statement-on-covid-19-astrazeneca-vaccine-advice-from-atagi
4. Therapeutic Goods Administration. AstraZeneca ChAdOx1-S COVID-19 vaccine – Updated safety advisory – rare and unusual blood clotting syndrome (thrombosis with thrombocytopaenia). Australian Government Department of Health. Available from: https://www.tga.gov.au/media-release/astrazeneca-chadox1-s-covid-19-vaccine
5. Heit JA, Spencer FA, White RH. The epidemiology of venous thromboembolism. Journal of thrombosis and thrombolysis 2016; 41(1): 3-14.

Authors
Micah D J Peters PhD and Mr Casey Marnie are in the National Policy Research Unit, Australian Nursing & Midwifery Federation (Federal Office).
Both are based at the University of South Australia, Clinical and Health Sciences, Rosemary Bryant AO Research Centre.