These COVID-19 vaccine FAQs have been designed to address common queries relating to Vaxzevria (AstraZeneca).

Please also see our other COVID-19 FAQs for more information on other COVID-19 related topics:

For ease of reference, information has been categorised as per the below themes:

This page will be updated on a regular basis as further information becomes available regarding COVID-19 vaccines.

For questions that have not been addressed on this page or our dedicated COVID-19 resource page, please contact us here.

Thrombosis with thrombocytopenia (TTS)

  • What is thrombosis with thrombocytopenia syndrome?

    Thrombosis with thrombocytopenia syndrome, is a rare and new syndrome which has been reported in people who have received an adenoviral vector COVID-19 vaccine (eg. Johnson & Johnson/Janssen and Vaxzevria (AstraZeneca)).

    The syndrome is distinct from other clotting conditions as it is characterised by thrombosis formation (blood clots) combined with thrombocytopenia (low platelet levels).  Symptoms occur 4-30 days following vaccination. Early recognition can lead to effective treatment.

  • What is the difference between Tier 1 and Tier 2 thrombosis with thrombocytopenia syndrome (TTS)?

    Tier 1 and Tier 2 definitions have been applied to TTS by the CDC (Centers for Disease Control and Prevention) in the United States. The tiers are defined by the following:

    Tier 1:

    • unusual site of thrombosis (e.g neurological – central venous sinus thrombosis [CVST] or gut – eg splanchnic vein, associated with bowel ischaemia and surgery, portal vein or other rare venous and arterial thromboses)
    • Platelet count <150,000 per microliter
    • Positive (+) heparin-P4 ELISA HIT antibody result is supportive, but not required

    Tier 2:

    • Usual sites of thrombosis such as leg or lungs (eg. venous thromboembolism, deep vein thrombosis, pulmonary embolism)
    • Platelet count <150,000 per microliter
    • Positive (+) heparin-P4 ELISA HIT antibody result is required

    Tier 1 is more severe, with higher morbidity and mortality than Tier 2. Evidence is emerging that Tier 1 is more common in younger age groups, hence the preferential recommendation for alternative vaccines (in Australia, Vaxzevria (AstraZeneca) is not the preferred vaccine for people aged < 60 years – see the Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca for further information).

  • Are there any risk factors for developing thrombosis with thrombocytopenia syndrome (TTS)? eg. age, gender etc

    There have been no known markers identified that either increase or decrease an individual’s risk of developing TTS. This includes having a past history of clots in the leg (DVT), lungs (pulmonary embolus) or heart (myocardial infarction).

    Evidence thus far indicates there is a higher risk of TTS in the younger population (< 60 years of age), although there has been a small number of cases identified in older adults.

  • When do symptoms of thrombosis with thrombocytopenia syndrome (TTS) occur?

    Symptoms of TTS have been reported to occur in the 4-30 day time period following administration of a COVID-19 adenoviral vector vaccine (eg. Johnson & Johnson/Janssen or Vaxzevria (AstraZeneca)). Current information suggests that the risk of TTS is much lower following receipt of a second dose.

    Symptoms can include

    • Cerebral venous sinus thrombosis (CVST): persistent headaches (unresponsive to simple analgesia), visual changes, focal neurological symptoms (e.g. movement or sensation changes), seizures, coma, secondary intra-cerebral haemorrhage, confusion, encephalopathy
    • Splanchnic vein thrombosis: abdominal pain
    • Pulmonary embolus: chest pain/significant respiratory symptoms or distress, dyspnoea
    • Deep vein thrombosis: leg pain, redness or swelling
    • Arterial ischaemia: pallor and coldness in limb, myocardial ischaemia
    • Thrombocytopenia: acute onset bruising or bleeding, petechial rash

    NB: These symptoms are different from the common or expected side effects following vaccination which usually occur in the first 24-48 hours and last 1-2 days.

  • I am over 60 years of age and have a history of deep vein thrombosis (DVT’s). Is it safe for me to receive the Vaxzevria (AstraZeneca) vaccine?

    There is currently no evidence to suggest that having a history of DVT’s or other general thromboembolic disorders predisposes you to developing thrombosis with thrombocytopenia syndrome (TTS) following receipt of an adenoviral vector vaccine such as Vaxzevria (AstraZeneca). In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    Refer to the Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca for more information.

  • I am taking hormone replacement therapy. Am I at an increased risk of thrombosis with thrombocytopenia syndrome (TTS) if I am given Vaxzevria (AstraZeneca)?

    There is currently no evidence to suggest that taking certain medications or being prone to developing blood clots puts you at increased risk of developing TTS following receipt of Vaxzevria (AstraZeneca). In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    For more information please refer to: Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca.

  • I am over 60 years of age and have a history of atrial fibrillation putting me at a higher risk of blood clots. Is it safe for me to have Vaxzevria (AstraZeneca)?

    There is currently no evidence to suggest that having a medical condition which increases your likelihood of developing blood clots puts you at a greater risk of developing TTS following receipt of Vaxzevria (AstraZeneca).

    There is currently no evidence to suggest that having a history of atrial fibrillation predisposes you to developing thrombosis with thrombocytopenia syndrome (TTS) following receipt of an adenoviral vector vaccine such as Vaxzevria. In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    Refer to the Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca for more information.

  • ATAGI have noted that “people of any age without contraindications who have had their first dose of Vaxzevria (AstraZeneca) vaccine without any serious adverse events should receive a second dose of the same vaccine``. What constitutes a serious adverse event?

    A serious adverse event following immunisation (AEFI) includes anaphylaxis or thrombosis with thrombocytopenia syndrome (TTS).

    Anaphylaxis to a previous dose of a vaccine is a contraindication to future doses of that same vaccine.

    As a precaution those with a past history of TTS, cerebral venous sinus thrombosis (CVST), heparin-induced thrombocytopenia (HIT), idiopathic splanchnic (mesenteric, portal and splenic) venous thrombosis, anti-phospholipid syndrome (with thrombosis and/or miscarriage), or capillary leak syndrome are advised not to receive Vaxzevria (AstraZeneca).

    All adverse events should be reported to SAEFVIC. Specialist immunisation advice can be sought by referring patients to the VicSIS.

General questions

  • Can Vaxzevria (AstraZeneca) be used for booster doses?

    Vaxzevria (AstraZeneca) is no longer recommended for use as a booster dose, with mRNA COVID-19 vaccines being the preferred choice (Comirnaty (Pfizer) or Spikevax (Moderna)). Individuals who have already received Vaxzevria (AstraZeneca) as their booster dose do not need a repeat booster dose of an alternate brand.

    Nuvaxovid (Novavax) may be given as a booster in individuals aged 12 years or older in circumstances where there is no other suitable vaccine.

    Using an alternate brand of vaccine for booster doses compared with the brand used to complete the primary course is acceptable.

  • I am under 60 years of age and have had dose 1 of Vaxzevria (AstraZeneca), is it safe to receive dose 2?

    People who have received their first dose of Vaxzevria (AstraZeneca) without any serious adverse events can receive their second dose. This should be done in the recommended time frame of 4-12 weeks between doses. Current information suggests that thrombosis with thrombocytopenia syndrome is more frequently reported following receipt of dose 1 of Vaxzevria.

    For more information refer to ATAGI statement on revised recommendations on the use of COVID-19 Vaccine AstraZeneca.

  • I am under 60 years of age and received Vaxzevria (AstraZeneca) as my first dose, should I/can I have an alternate brand to complete the course?

    Where possible ATAGI recommends that a COVID-19 vaccine course is completed with the same brand. Combined or mixed schedules are not routinely recommended in Australia however there is emerging evidence relating to the safety and efficacy of using alternate brands.

    Current information suggests that thrombosis with thrombocytopenia syndrome (TTS) is rarely reported following receipt of dose 2 of adenoviral vector vaccines. If a person has received the first dose of Vaxzevria (AstraZeneca) without any serious adverse events such as TTS or anaphylaxis, then they can receive the second dose.

  • I am over 60 years of age and have a history of deep vein thrombosis (DVT’s). Is it safe for me to receive Vaxzevria (AstraZeneca) vaccine?

    There is currently no evidence to suggest that having a history of DVT’s or other general thromboembolic disorders predisposes you to developing thrombosis with thrombocytopenia syndrome (TTS) following receipt of an adenoviral vector vaccine such as Vaxzevria (AstraZeneca). In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    Refer to the Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca for more information.

  • I am under 60 years of age and travelling to an area experiencing a current COVID-19 outbreak. Should I have the Vaxzevria (AstraZeneca) vaccine as I am unable to source an alternative?

    ATAGI recommends that Vaxzevria (AstraZeneca) can be administered in adults aged under 60 years of age where the benefits of protection are likely to outweigh the risks of vaccination for that individual. It is important to discuss your individual circumstances with a health care provider in order to make an informed decision. There may be some differences to this advice depending on the state/jurisdiction you live in, so please check with your local Department of Health.

    To read more refer to ATAGI statement on revised recommendations on the use of COVID-19 Vaccine AstraZeneca and ATAGI statement on use of COVID-19 vaccines in an outbreak setting.

  • I am taking hormone replacement therapy. Am I at an increased risk of thrombosis with thrombocytopenia syndrome (TTS) if I am given Vaxzevria (AstraZeneca) vaccine?

    There is currently no evidence to suggest that taking certain medications or being prone to developing blood clots puts you at increased risk of developing TTS following receipt of Vaxzevria (AstraZeneca) vaccine.  In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    For more information please refer to: Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca.

  • I am over 60 years of age and have a history of atrial fibrillation putting me at a higher risk of blood clots. Is it safe for me to have Vaxzevria (AstraZeneca) vaccine?

    There is currently no evidence to suggest that having a medical condition which increases your likelihood of developing blood clots puts you at a greater risk of developing TTS following receipt of Vaxzevria (AstraZeneca) vaccine.

    There is currently no evidence to suggest that having a history of atrial fibrillation predisposes you to developing thrombosis with thrombocytopenia syndrome (TTS) following receipt of an adenoviral vector vaccine such as Vaxzevria. In contrast, there is strong evidence that COVID-19 disease is thrombogenic (promotes clot development) and may cause a variety of thromboembolic events. TTS is a unique condition involving the development of thromboses (blood clots) combined with thrombocytopenia (low platelets).

    Refer to the Joint statement from ATAGI and THANZ on Thrombosis with Thrombocytopenia Syndrome (TTS) and the use of COVID-19 Vaccine AstraZeneca for more information.

  • I am 61 years of age. Why is Vaxzevria (AstraZeneca) vaccine deemed safe for me to receive?

    Like any arbitrary age cut-off, it is acknowledged that there may be limited difference between an individual at age 60 or 61 years. What we do know is that the risk of intensive care admissions and death increases markedly with age, with each decade of life increasing the risk 3-fold. Conversely, the possibility of thrombosis with thrombocytopenia syndrome (TTS) may be higher in younger people, based on currently available data.

    ATAGI have taken this data, along with safety data analysis from other countries such as the UK, other risks such as prevalence of COVID-19 in the Australian population along with any existing COVID-19 outbreaks, to calculate an age-specific benefit-to-risk balance for the Australian population. This has resulted in a current age cutoff of 60 years old in the current recommendations.

  • Can I get a blood test to check my immune response from the first dose and avoid the need for a second dose?

    COVID-19 serology is not routinely available following vaccination and is not able to inform the decision to proceed with future doses. Due to the novel nature of SARS-CoV-2, a correlate of protection has not yet been established for COVID-19 in humans.

    Data from clinical trials demonstrated that a 4-12 week interval between 2 doses of Vaxzevria (AstraZeneca) vaccine was needed to provide long-term protection.

  • How much protection do I get from 1 dose of Vaxzevria (AstraZeneca)?

    The level of protection gained from a single dose of Vaxzevria (AstraZeneca) was assessed during clinical trials in an exploratory analysis that included participants who had received one dose. Protection was 73%, with the 95% CI from 49% and 86%, starting from 3 weeks after the first dose. This analysis reflects the short term efficacy and does not demonstrate the duration of protection. It is important to note that Vaxzevria is provisionally licensed by the Therapeutic Goods Administration as a two dose schedule.

    Refer to the COVID-19 AstraZeneca Product Information for more information.

  • ATAGI have noted that “people of any age without contraindications who have had their first dose of Vaxzevria (AstraZeneca) without any serious adverse events should receive a second dose of the same vaccine``. What constitutes a serious adverse event?

    A serious adverse event following immunisation (AEFI) includes anaphylaxis or thrombosis with thrombocytopenia syndrome (TTS).

    Anaphylaxis to a previous dose of a vaccine is a contraindication to future doses of that same vaccine.

    As a precaution those with a past history of TTS, cerebral venous sinus thrombosis (CVST), heparin-induced thrombocytopenia (HIT), idiopathic splanchnic (mesenteric, portal and splenic) venous thrombosis, anti-phospholipid syndrome (with thrombosis and/or miscarriage), or capillary leak syndrome are advised not to receive Vaxzevria (AstraZeneca).

    All adverse events should be reported to SAEFVIC. Specialist immunisation advice can be sought by referring patients to the VicSIS.

  • I have a history of capillary leak syndrome, can I have the Vaxzevria (AstraZeneca)?

    An extremely rare relapsing-remitting condition known as capillary leak syndrome has been reported overseas following vaccination with Vaxzevria (AstraZeneca). The syndrome results in fluid leaking from capillaries (small blood vessels) into surrounding tissue and can lead to severe organ damage or death if left untreated.

    In two of the reported cases there was a previous history of capillary leak syndrome. As triggers for relapse are not well understood, the manufacturer of Vaxzevria has updated the product information advising it is not recommended that the vaccine be administered to people with a history of capillary leak syndrome. Individuals with a history of capillary leak syndrome should be referred to their closest VicSIS clinic for further assessment.

    Further information can be found at TGA: COVID-19 weekly safety report.

  • Is there an association with Vaxzevria (AstraZeneca) and developing myocarditis/pericarditis following vaccination?

    Myocarditis is an inflammatory disease of the heart muscle, whilst pericarditis is an inflammatory disease of the lining of the heart muscle. They are rare conditions, most commonly associated with viral infections (including SARS-CoV-2) but can also be triggered by other factors such as medications and autoimmune conditions.

    Vaxzevria (AstraZeneca) has been associated with a small increased risk of developing myocarditis/pericarditis following COVID-19 vaccination however this risk appears lower than that seen in individuals who have received Spikevax (Moderna) and Comirnaty (Pfizer).

Authors: Daniela Say (MVEC Immunisation Fellow), Georgina Lewis (SAEFVIC Clinical Manager, Murdoch Children’s Research Institute), Rachael McGuire (MVEC Education Nurse Coordinator) and Francesca Machingaifa (MVEC Education Nurse Coordinator)

Reviewed by: Rachael McGuire (MVEC Education Nurse Coordinator)

Date: October 6, 2022

Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family’s personal health. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult a healthcare professional.