ATAGI met on Wednesday 7 July 2021 to review the latest developments relating to the AstraZeneca COVID-19 vaccine and Thrombosis and Thrombocytopenia Syndrome (TTS) cases in Australia. In addition, ATAGI continues to monitor COVID-19 epidemiology, vaccine coverage and adverse events observed following immunisation.
ATAGI considered an update from the Therapeutic Goods Administration (TGA) on current confirmed cases of TTS and those under investigation. The latest TGA statement on TTS cases, including clinical outcomes, can be found here.
ATAGI examined estimates of risk of TTS by age group in Australia and note that there have been 76 cases of confirmed or probable TTS (45 confirmed cases; 31 probable cases). To date around 5 million doses of COVID-19 Vaccine AstraZeneca have been administered. As of 23 June 2021, approximately 4.5 million doses of COVID-19 Vaccine AstraZeneca have been administered, made up of around 4.1 million first doses and 400,000 second doses. All rates of TTS cases are based on first doses of COVID-19 Vaccine AstraZeneca to 23 June 2021, which allows for delays in case reporting.
Although estimates of risk based on small numbers of cases are imprecise, the risk of TTS is estimated in Australia at around:
- 2.6 per 100,000 in those <60 years; and
- 1.6 per 100,000 in those ≥60 years.
A breakdown of current rates by decade of age for those aged < 60 & ≥ 60 years is included here:
|Age bracket (years)
ATAGI also noted that the TGA has reviewed Australia’s confirmed and probable TTS cases and those reported by overseas regulators using the recently proposed CDC Criteria, which uses the following categories:
- Tier 1: criteria are defined as clots in an unusual location such as the brain or abdomen and a low platelet count with or without a positive test for antibodies that activate platelets (anti-PF4 antibodies);
- Tier 2: criteria are defined as only clots found in more usual locations such as the legs or lungs with a low platelet count and a positive test for anti-PF4 antibodies.
When considering Australian TTS cases in using the recently proposed CDC Criteria, ATAGI noted in the Australian context:
- 29 confirmed and probable TTS cases met the CDC Tier 1 definitions. 15 of which occurred in those younger than 60 years; and
- 21 confirmed and probable TTS cases met the CDC Tier 2 definitions.
- 26 confirmed and probable TTS cases do not meet either CDC Tier 1 or 2 definitions. These include cases with clots in common locations with thrombocytopenia but no evidence of anti-PF4 antibodies, including some with arterial thrombosis. Cases may be reclassified as more clinical data are received.
ATAGI notes that there was a higher proportion of tier 1 cases (which are generally associated with increased morbidity) in those under 60 years of age. 54% of TTS cases in people under 60 are tier 1, compared with 29% of TTS cases in people over 60 years.
ATAGI was encouraged by data demonstrating uptake of second doses of COVID-19 Vaccines, including COVID-19 Vaccine AstraZeneca. ATAGI is continuing to closely monitor local and international data on TTS cases and notes risk of TTS following a second dose of COVID-19 Vaccine AstraZeneca is much lower than the risk following a first dose (estimated internationally to be 1.6 per million second doses). In the second doses of COVID-19 Vaccine AstraZeneca administered to date, there have been no confirmed or probable cases of TTS. ATAGI reinforced the importance of completing a two-dose schedule with the same brand to ensure maximal protection.
ATAGI emphasised that the ATAGI/THANZ joint statement on TTS and the use of the COVID 19 Vaccine AstraZeneca is an important resource. The statement provides updated information about TTS and reaffirms ATAGI’s previous advice regarding the safe use of the AstraZeneca COVID-19 Vaccine.
At this time, there is no update to the ATAGI statement from 17 June 2021 in relation to the use of COVID-19 Vaccine AstraZeneca.
ATAGI recommends the COVID-19 Pfizer vaccine (Comirnaty) as the preferred vaccine for those aged 16 to under 60 years. For those aged 60 years and above, the individual benefits of receiving a COVID-19 vaccine are greater than in younger people. The risks of severe outcomes with COVID-19 increase with age and are particularly high in older unvaccinated individuals. The benefit of vaccination in preventing COVID-19 with COVID-19 Vaccine AstraZeneca outweighs the risk of TTS in this age group and underpins its ongoing use in this age group. ATAGI emphasises the risk-benefit document is an important resource to help consumers make informed decisions.
ATAGI is closely monitoring reports of other rare but potentially serious adverse events following immunisation, including myocarditis following Comirnaty and Immune Thrombocytopenia following COVID-19 Vaccine AstraZeneca.