COVID-19 vaccine weekly safety report – 17-06-2021

COVID-19 vaccine weekly safety report – 17-06-2021


The TGA and other medicines regulators around the world continue to closely monitor and investigate thrombosis with thrombocytopenia syndrome (TTS). This is a rare event involving serious blood clots with a low blood platelet count. TTS is triggered by the immune system’s response to the AstraZeneca vaccine and is different from other clotting conditions. 

As previously reported, the TGA determines whether a report is likely to represent TTS by assessing cases against a consistent set of criteria, based on the case definitions established by the UK’s Medicines and Healthcare products Regulatory Agency.

Since last week’s report, a further 12 reports of blood clots and low blood platelets have been assessed as confirmed or probable TTS likely to be linked to the AstraZeneca vaccine (Table 1). 

Table 1: Newly confirmed and probable TTS cases for the week of 11-17 June 2021

New confirmed TTS

New probable TTS

Three new cases:


Nine new cases:

  • 54-year-old man from Northern Territory

  • 65-year-old woman from Tasmania

  • 50 and 56-year-old men and a 69-year-old woman from Victoria

  • 58-year-old woman from South Australia

  • 59 and 80-year-old men from Queensland

  • 67-year-old woman from NSW

This takes the total Australian reports assessed as TTS following the AstraZeneca vaccine to 37 confirmed cases and 23 probable cases. 

Most cases have occurred in people aged over 50 years because the AstraZeneca vaccine has been used almost exclusively in this age group since the recommendation from ATAGI on 8 April 2021 that the Comirnaty vaccine is preferred in people under 50. 

The TGA’s adjudication committee, which met to review these cases on 16 June 2021, confirmed that seven of the 12 new cases this week were in individuals aged 50–59 years old. 

When assessed against the criteria used by the US Centers for Disease Control and Prevention (CDC), fewer than half of them are classified as Tier 1 cases, which involve clots in an unusual location, such as the brain or abdomen (Table 2). Of note, Tier 1 cases tend to have more serious outcomes than Tier 2 cases.

Table 2: Confirmed and probable TTS cases by age and CDC classification


Total cases

CDC classification†

Tier 1

Tier 2

Not classified

<30 years





























All ages


(29 men, 31 women)




†The US CDC classification is defined as:

  • Tier 1 = clots in an unusual location (such as the brain or abdomen) and a low platelet count with or without antibodies that activate platelets (anti-PF4 antibodies)

  • Tier 2 = clots found in common locations (such as the leg or lungs) and a low platelet count and anti-PF4 antibodies

  • Not classified = case does not meet the criteria for Tier 1 or Tier 2 (for example clots in common locations with low platelet count but no evidence of anti-PF4 antibodies).

Cases have most often occurred about two weeks after vaccination, although the time to onset (or diagnosis) has ranged from two days to 52 days (Table 3). In some cases with a longer time to diagnosis, patients had experienced symptoms at an earlier stage but complicating factors, including symptoms from comorbidities, may have delayed a clear diagnosis. Approximately one in four TTS cases has required Intensive Care Unit (ICU) treatment, although all but four patients have since been released from ICU.

Table 3: Time to onset, treatment and outcomes for TTS cases*

Time to onset/ diagnosis (days) Median (range) 13 (2-52)
Treated in ICU At any point 13
Currently 4
Outcome Discharged 36
In hospital 22
Fatal 2

*Data is based on the most recent medical information available to the TGA

Updated reporting rates of TTS in Australia were published in last weeks’ statement from ATAGI published 10 June 2021. The rates remain consistent with what is being seen internationally, including in Europe, the UK, the Middle East and Canada. 

While TTS is very rare, it is appreciated that some people will may have concerns that they can discuss with their doctor. This is essential to allow people to make an informed choice. 

There have only been a limited number of second doses of AstraZeneca vaccine administered so far in Australia. Data from the UK indicates that TTS is much less common after second doses, with an overall incidence of 1.5 cases per million doses.

Anyone who has been vaccinated should seek immediate medical attention if they develop any of the following symptoms after vaccination:

  • severe or persistent headache or blurred vision

  • shortness of breath, chest pain, leg swelling or persistent abdominal pain

  • unusual skin bruising and/or pinpoint round spots beyond the site of vaccination.

The most common time period for onset of TTS symptoms is 4-30 days after vaccination.


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