The Australian Government receives advice and recommendations from the Australian Technical Advisory Group on Immunisation (ATAGI) about the AstraZeneca vaccine. 


There has been a link between the AstraZeneca COVID-19 vaccine and a rare condition called thrombosis with thrombocytopenia syndrome (TTS). This condition appears to be more common in younger adults.


Comirnaty (Pfizer) is the preferred COVID-19 vaccine for adults under 50 years of age at this time. However, adults under 50 years of age may still choose to receive the AstraZeneca COVID-19 vaccine if they have weighed up the benefits and the risks. Talk to your doctor or immunisation provider to help inform your decision.


All adults are recommended to be vaccinated against COVID-19. The risk of severe illness and death from COVID-19 progressively increases with age in older adults. This means that older adults will have a higher benefit from vaccination. 
More information on what this means can be found at www.health.gov.au/covid19-vaccines


What is thrombosis with thrombocytopenia syndrome (TTS)?
This is a rare side effect of the AstraZeneca COVID-19 vaccine. It can be very serious and can cause long-term disability and death. Comirnaty (Pfizer) is not associated with a risk of TTS.
The condition causes thrombosis (blood clotting) and thrombocytopenia (low blood platelet counts). It is different from general clotting disorders such as deep vein thrombosis (DVT) or pulmonary embolism (PE). TTS can occur at different parts of the body, including the brain (called cerebral venous sinus thrombosis) and in the abdomen (idiopathic splanchnic vein thrombosis). The low level of blood platelets can potentially cause bleeding. 
•    Thrombosis is the formation of a blood clot, which prevents blood flowing normally through the body. While thrombosis is usually a normal response to prevent bleeding (e.g. following injury), in this case this process is abnormal.
•    Thrombocytopenia is a condition in which you have a low blood platelet count. Platelets (thrombocytes) are blood cells that help blood clot. Platelets stop bleeding by clumping and forming plugs in injured blood vessels. 


When does TTS typically occur?
TTS typically occurs around 4 to 28 days after vaccination with AstraZeneca COVID-19 vaccine. 
How common is TTS?
TTS is rare. It is currently estimated to affect about 1-2 per 100,000 people who receive the AstraZeneca COVID-19 vaccine. But for those under 50 years of age, the rate is currently estimated to be higher. These estimates are based on the small numbers of people who have been vaccinated in Australia but are similar to rates seen in some countries overseas. They will be updated as further information become available.


Are any groups more at risk of TTS?
The rate of TTS reported in Australia and overseas is higher in younger adults and it may be more common in women. However cases have also been reported in men and in older people. It is not yet clear if women are at higher risk. More women than men have been vaccinated in some countries as they are a large proportion of frontline healthcare workers and have been prioritised for vaccination.
Based on current information, we do not know if there are any pre-existing medical conditions that may contribute to developing TTS or make it worse if it occurs. 


What symptoms does TTS usually cause?
If you experience the following symptoms after vaccination you should seek medical attention immediately:
•    A severe persistent headache with additional features:

o    appears at least 4 days after vaccination

o    does not improve with simple painkillers
o    may be worse when lying down
o    may be accompanied by nausea and vomiting
•    Neurological symptoms such as:
o    blurred vision
o    difficulty with speech
o    drowsiness
o    seizures 
•    Shortness of breath
•    Chest pain
•    Swelling in your leg
•    Persistent abdominal (belly) pain
•    Tiny blood spots under the skin away from the site of injection.


Do the benefits of the AstraZeneca vaccine outweigh any risk?
Yes. The benefits of vaccination clearly outweigh the risks in many circumstances. This is particularly so for older people who have a higher risk of severe illness or death from COVID-19.  
Cases of TTS have occurred in people of all ages. However, the risk of TTS appears to be lower in those over 50 years old than in younger adults.
For people under 50 years of age, Comirnaty (the Pfizer COVID-19 vaccine) is preferred. However, adults under 50 may still choose to have AstraZeneca vaccine after considering the benefits and risks of vaccination. The risks of dying or having severe disease from COVID-19 are generally lower in healthy younger adults and they have a higher (although still rare) risk of TTS after vaccination compared with older adults. 
What if you are aged 50 and under and are booked in to receive your first dose of AstraZeneca?
If you are under the age of 50, Comirnaty (the Pfizer vaccine) is preferred for you. If you are booked in to receive your first dose of the AstraZeneca COVID-19 vaccine and you are younger than 50 years old, please talk to your health professional


How do people aged under 50 years book an appointment for the Pfizer vaccine? Where is it available?
The COVID-19 vaccination program has been modified in light of the ATAGI advice. More Pfizer vaccination sites will become available around the country in line with increasing supplies. You can check your eligibility and where to get a vaccine using the eligibility checker.


What if you have received your first dose of AstraZeneca and are due for your second dose?
People who have received their first dose without any serious adverse events can be confident in getting their second dose of the of AstraZeneca COVID-19 vaccine. UK data suggests that the risk of TTS is much lower after the second dose, with 15 cases reported to date out of 9.0 million second doses of the AstraZeneca COVID-19 vaccine given. This translates into an estimated rate of 1.7 cases per million doses.  


What about people with heparin induced thrombocytopenia (HIT), central venous sinus thrombosis (CVST), idiopathic splanchnic vein thrombosis or antiphospholipid syndrome with thrombosis?
TTS is different to these certain rare conditions, although there are similarities between TTS and each of these conditions. As a precaution, in people with a history of heparin induced thrombocytopenia, central venous sinus thrombosis, idiopathic splanchnic vein thrombosis or antiphospholipid syndrome with thrombosis, the Pfizer (Comirnaty) vaccine is recommended over the AstraZeneca COVID-19 vaccination at this time. The Pfizer (Comirnaty) vaccine is not associated with TTS. 


Is the AstraZeneca vaccine safe in people who have had blood clots in the past?
If you have had other types of blood clots in the past, or if you have risk factors for blood clots, you can still have the AstraZeneca vaccine. There is no evidence that people who have had a past history of other types of blood clots have an increased risk of developing TTS or becoming more ill from it if it occurs.

 
People with the following conditions can receive the AstraZeneca COVID-19 Vaccine:
·    History of blood clots in typical sites
·    Increased clotting tendency that is not immune-mediated
·    Family history of blood clots
·    History of ischaemic heart disease or stroke
·    Current or past thrombocytopenia (low platelet count)
·    Those receiving anticoagulation therapy


The overall rate of blood clots has not risen in countries which have extensively used the AstraZeneca vaccine with millions of doses administered. Blood clots occur commonly in the population, and not all blood clots that occur after AstraZeneca COVID-19 will be caused by the vaccine. If you develop a blood clot after vaccination, your doctor can do blood tests to determine the cause.
Speak to your healthcare professional or vaccination provider if you have further questions about the AstraZeneca vaccine.