June 17, 2021
The Australian Technical Advisory Group on Immunization (ATAGI) recommends the Pfizer (Comirnaty) COVID-19 vaccine as the vaccine of choice for those aged 16 to under 60 years. This updates previous preferential recommendations for Comirnaty over COVID-19 Vaccine AstraZeneca in those aged 16 to under 50 years. The recommendations were revised due to the higher risk and observed severity of the thrombosis and thrombocytopenia syndrome (TTS) associated with the use of the AstraZeneca COVID-19 vaccine observed in Australia in the 50-59 year age group than was reported internationally and initially estimated in Australia.
For those aged 60 and over, the individual benefits of receiving the COVID-19 vaccine are greater than for younger people. The risk of a severe outcome with COVID-19 increases with age and is particularly high in unvaccinated older individuals. The benefit of vaccination in preventing COVID-19 with AstraZeneca’s COVID-19 Vaccine outweighs the risk of TTS in this age group and supports its continued use in this age group.
People of all ages without contraindications who have received the first dose of the AstraZeneca COVID-19 Vaccine without serious side effects should receive a second dose of the same vaccine. This is supported by data showing significantly lower TTS levels after AstraZeneca’s second dose of COVID-19 vaccine in the United Kingdom (UK).
Australia’s COVID-19 vaccination program has the overarching goal of protecting everyone in Australia from the harm caused by the new coronavirus SARS-CoV-2.
On April 8, 2021, ATAGI recommended that Comirnaty be the vaccine of choice for people under the age of 50 due to local and international reports of thrombosis and thrombocytopenia syndrome (TTS) following the AstraZeneca COVID-19 Vaccine.
Based on international data available at the time, the estimated risk of TTS was 4-6 per million cases after the first dose of AstraZeneca’s COVID-19 Vaccine. Given the continued risk of COVID-19 outbreaks, low vaccine coverage, and increased rates of severe COVID-19 outcomes in older people, the benefits of AstraZeneca’s COVID-19 Vaccine are considered to outweigh the risks in those over 50 years of age. Thus, no preference recommendation for either vaccine is made in this age group. This suggestion was strengthened on April 23, 2021 and has been reviewed weekly by ATAGI since then.
Principles underlying the revised recommendations
In making the decision to revise the previous recommendation, ATAGI has considered several factors that have been closely monitored, including:
- Potential risk of severe illness and death from COVID-19 in the coming months
- Minimize harm to people due to side effects after immunization
- Australian data on age-specific risk and severity of TTS after AstraZeneca COVID-19 Vaccine
- Expected supply of vaccines over the next few months
- The impact of any changes to recommendations on the COVID-19 vaccine program.
The benefits of vaccination to prevent COVID-19
There is an ever-present risk of COVID-19 in Australia while a large part of the population remains vulnerable to infection. Recent events in Victoria have demonstrated how quickly an outbreak can spread despite intensive contact tracing and public health measures. As of June 16, 2021, 63% of people aged 70 and over and 25% of those aged 18 and over had received at least one dose of the COVID-19 vaccine.
The risk of severe COVID-19 is strongly associated with increasing age. By 2020, for every 100 people with COVID-19 aged between 50-59 years, about 14 are hospitalized and 3 require treatment in an intensive care unit (ICU). One in every 600 people with COVID-19 in this age group dies. In contrast, for every 100 people aged 70-79 years with COVID-19, approximately 38 were hospitalized, 7 were admitted to the ICU and 4 died (i.e. 24 deaths in 600). Therefore, the benefit of vaccination in preventing COVID-19 is greater in the elderly. If an outbreak were to occur comparable to the first wave in Australia, the benefits in preventing severe COVID-19 would outweigh the risk of TTS due to the AstraZeneca COVID-19 Vaccine in older adults, as illustrated in Weighing the potential benefits against the risks of harm from the COVID Vaccine -19 AstraZeneca.
ATAGI recognizes the difficulty in balancing the small risk of clinically significant adverse events associated with vaccination with the AstraZeneca COVID-19 Vaccine against the need to protect individuals and communities against the ongoing threat of COVID-19, together with the limitations and continuing uncertainty about the provision of alternative vaccines. COVID-19. ATAGI emphasizes that this advice is context specific in that there is currently no or limited community transmission in most of Australia and will be different in other countries.
TTS risk after AstraZeneca’s COVID-19 Vaccine
From early April to 16 June 2021, 60 confirmed or probable cases of TTS have been reported in Australia. This includes an additional seven cases reported in the past week in people aged 50-59 years, increasing the rate in this age group from 1.9 to 2.7 per 100,000 doses of AstraZeneca vaccine. The revised risk estimates associated with the first dose of AstraZeneca COVID-19 Vaccine are listed in the table below.
|Age||Estimated TTS risk per 100,000|
AstraZeneca vaccine dose (first dose)
50-59 years old
60-69 years old
70-79 years old
TTS is a serious condition in some individuals who develop it. Australia’s overall case fatality rate (3%; 2 deaths among 60 cases) is lower than internationally reported. This likely reflects increased detection due to increased awareness, as well as early diagnosis and treatment. A spectrum of disease severity has been reported in Australia, from fatal cases and cases with significant morbidity, to relatively mild cases. TTS seems to be more severe in younger people.
There are various ways in which the severity of TTS can be measured. The US Centers for Disease Control and Prevention (CDC) defines a “grade 1” case as a clot involving an unusual site, such as a blood vessel to the brain (cerebral venous sinus thrombosis) or abdomen (splanchnic thrombosis); these are generally more severe and have the potential to cause long-term health complications. In those under 60 years of age, 52% of TTS episodes occurred at tier 1 sites compared with 28% in those 60 years and older. Other markers of severity include the need for intensive care (33% of TTS in those under 60 years; 15% of TTS cases in those 60 years and over), and fatal cases (both occurring in those with
Recommended second dose for AstraZeneca’s COVID-19 Vaccine
ATAGI supports completion of a two-dose schedule with AstraZeneca’s COVID-19 Vaccine, based on current evidence. The risk of TTS after the second dose of the AstraZeneca COVID-19 Vaccine is much lower than the risk after the first dose. The UK has reported 23 cases of TTS in 15.7 million people after receiving the second dose, an estimated rate of 1.5 per million second dose (compared to a reported risk of 14.2 per million first dose in the UK).
People of all ages without contraindications who have received the first dose of AstraZeneca COVID-19 Vaccine without serious side effects should receive a second dose.
- ATAGI recommends that Comirnaty be preferred over AstraZeneca’s COVID-19 Vaccine from ages 16 to under 60 years. This is based on the most recent data on TTS cases in Australia and a reassessment of the current age-specific risks and benefits of vaccination.
- ATAGI considers that the benefit of vaccination in preventing COVID-19 with the AstraZeneca COVID-19 Vaccine is greater than the risk of TTS in people aged 60 years and over. For this age group, the benefits of receiving the COVID-19 vaccine are greater than for younger people. The risk of a severe outcome with COVID-19 increases with age and is particularly high in unvaccinated older individuals.
- AstraZeneca’s COVID-19 vaccine can be used in adults under 60 years of age who do not have Comirnaty, the benefits are likely to outweigh the risks to the individual and the person has made their decision based on an understanding of the risks and benefits.
- People of all ages without contraindications who have received the first dose of AstraZeneca COVID-19 Vaccine without serious side effects should receive a second dose.
- ATAGI stresses the importance of providing clear communication to people who have received or is considering the AstraZeneca COVID-19 Vaccine, and notes that the guidance documents for consumers, for primary care and for hospitals are constantly being revised to accommodate these new recommendations.
The next step
ATAGI continues to monitor the evidence regarding the risk of TTS and the epidemiology of COVID-19, and will continue to review recommendations. Further modifications may be recommended as additional COVID-19 vaccine supply and emerging evidence become available. ATAGI emphasized that due to the continued risk of COVID-19, maximizing vaccine coverage is a priority, especially for those who are at greatest risk of severe COVID-19.
ATAGI is currently working with general practitioners, emergency physicians and hematologists to update clinical advice on TTS for consumers and primary care.