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Trans Vaccine Safety Message

How does AstraZeneca affect trans people on gender affirming hormones?

Trans people on gender-affirming hormones are not at higher risk of blood clots with the AstraZeneca COVID-19 vaccine

By Dr Ada Cheung – Endocrinologist, Trans and Gender Diverse in Community Health consortium, Victoria and Trans Health Research Group, The University of Melbourne.

Download the printable PDF here: Fact Sheet: HRT & COVID-19 vaccines

Many trans people (including people with binary and/or non-binary identities) may be wary about getting vaccinated against COVID-19, particularly as medical institutions have often pathologized trans and gender non-conforming people. Messaging has been complex, frequently changing and at times misinformed. Moreover, the word “blood clots” can cause alarm for many trans people on gender affirming hormone therapy and lead to hesitancy to consider the AstraZeneca vaccine.

Let’s tackle some common questions specifically around gender affirming hormones and COVID-19 vaccines.


I’m on HRT. Am I at higher risk of blood clots with AstraZeneca?

The AstraZeneca vaccine is associated with a rare side-effect called thrombosis with thrombocytopenia syndrome (TTS). This is different to the more common types of blood clots (deep vein thrombosis or pulmonary embolus) that are known to occur more often in people using estradiol or progestin therapy. Blood clots in people using estradiol therapy are uncommon and thought to be due to increased liver coagulation “sticky” factors and are typically treated with blood thinners. TTS is different from other clotting conditions. TTS is triggered by an immune response to the AstraZeneca vaccine causing the platelet levels (the blood cells responsible for clotting) levels to drop (thrombocytopenia), malfunction and in turn clot, typically affecting different parts of the body such as the brain (cerebral venous thrombosis) or abdomen (portal vein thrombosis). The mechanism of TTS is not fully understood but it is an immune-driven disease that causes platelet overactivity. TTS is treatable with blood thinners and intravenous immunoglobulins are used in more severe cases.

The rate of TTS appears to be higher in younger adults but there is no evidence to suggest that anyone on estrogen or progestin therapy, anyone on testosterone therapy or even anyone who has had previous blood clots are at higher risk of TTS with the AstraZeneca vaccine. The only caution is in people who have had the very rare heparin-induced thrombocytopenic thrombosis, antiphospholipid antibody syndrome with blood clots, or people who have had cerebral venous thrombosis or abdominal vein thrombosis in the past. Based on real world data from the many hundreds of millions of AstraZeneca vaccines administered, people with other chronic conditions, using the contraceptive pill, having a history of cancer or history of blood clots face no additional risks. Being on gender-affirming hormones does not appear to increase the risk of TTS but note that due to small number of cases of TTS, it has not been specifically studied in the trans population. When we don’t have specific data, we often draw upon first principles, and being on estrogen would be unlikely to increase the immune reaction that leads to TTS.


How common are blood clots?

Research has explored the risks of COVID vaccines and the rare blood clotting condition cerebral venous thrombosis (CVT). CVT is higher in people having the AstraZeneca vaccine than the general population but there is also a slightly higher risk with people having Pfizer or Moderna COVID vaccines. Notably, having COVID-19 infection itself is associated with 8 – 10 times greater likelihood of developing CVT than those who have been vaccinated against it. In addition to the risk of CVT, COVID-19 infection also appears to increase the risk of other brain conditions 6 months after diagnosis and a range of long-term sequelae.


Reported Cases of CVT graph

Incidence of cerebral venous thrombosis (CVT) with COVID-19 infection, COVID-19 vaccines and the general population reproduced from


Should I wait for Pfizer?

This is a very common question being asked around Australia. Real-world data suggests that both Pfizer and AstraZeneca vaccines are equally effective, with no real difference in the level of protection offered. Having two doses offers much greater protection than a single dose of the vaccine. Both Pfizer and AstraZeneca vaccines are effective against the Delta variant. A recent large study showed that Pfizer is 88% effective against Delta compared to 93% for Alpha, and AstraZeneca 67% effective against Delta and 74.5% for Alpha. Real world data also shows that both Pfizer and AstraZeneca prevent severe disease with reduction in the risk of hospitalisation due to COVID by up to 97% with no difference between the two vaccines.

On the 17th June 2021, the Australian Technical Advisory Group on Immunisation (ATAGI) recommended that Pfizer be the preferred vaccine for people under 60, but supply of Pfizer has been extremely limited to-date. More supply is due to arrive in Australia in the next few months, but AstraZeneca is available now. There is enough AstraZeneca in Australia now for anyone who wants it.


Do the benefits outweigh the risks?

Both vaccines have clear benefits in reducing the risk of infections and reducing deaths or hospitalisations due to COVID-19. In areas where there are significant outbreaks of the COVID-19 delta variant, the risks of being infected with COVID-19 are higher, which shifts the risk to benefit ratio. The health benefits increase when the risk of exposure to COVID-19 is higher (see “Weighing up the potential benefits against risk of harm from COVID-19 Vaccine AstraZeneca”). In addition to health risks, there are also economic and social risks to consider. People aged 18 to 59 living in areas experiencing an outbreak should carefully consider the benefits of AstraZeneca vaccination weighed with the risks of rare but potentially serious side-effects (which are not increased by gender-affirming hormone therapy).

Does Pfizer cause menstrual bleeding in trans people on testosterone therapy?

There have been some anecdotal reports that cis and trans people have experienced changes to their menstrual cycles after having a COVID (particularly Moderna) vaccine. The Pfizer vaccine in Australia is an mRNA vaccine similar to Moderna. There have been trans people on testosterone therapy who don’t usually experience periods reporting that they have had some temporary spotting or bleeding or period pain after a vaccine. The link is not proven but a research study is underway to explore this. We know that the endometrium (uterus lining) can be affected by immune changes so there is a plausible link. If the vaccine does cause some changes to periods, these are usually temporary.


What if my name is different to the name on my Medicare card?

It can be challenging for some trans people to inform staff of their name, particularly if it is different to the name on your Medicare card. The process for changing your name and gender marker on your Medicare card is explained on Transhub (and is relevant across Australia). If you don’t feel comfortable telling health services staff, you may like to give them a letter requesting that they affirm your name (form available for download from Transhub here).


How do I get a vaccine?

Speak to your community leaders, GP, pharmacist, local health district or visit

Vaccination aims to protect you, your loved ones and everyone else in the community. We know that the trans community in Australia have been disproportionately negatively affected by social restrictions and the COVID-19 pandemic. Getting vaccinated will help reduce the health, social and economic impacts on our community.

If you have any questions specifically regarding COVID vaccines for your health situation, please have a chat to your local GP or email us at [email protected] and we will endeavour to respond to frequently asked questions.


Disclaimer: This is general advice only. The COVID-19 pandemic and investigations into the AstraZeneca vaccine and blood clots are rapidly evolving situations. This information may be superseded by later findings.