Thrombosis with thrombocytopenia syndrome (TTS) is a very rare event involving serious blood clots with a low blood platelet count. Emerging evidence suggests it is caused by the AstraZeneca vaccine. The TGA and other medicines regulators around the world continue to monitor and investigate this issue.

In response to this safety concern, ATAGI has recommended that Comirnaty (Pfizer) be preferred over the AstraZeneca vaccine in people aged under 50. This week the TGA has approved further changes to the Product Information to inform health professionals about recognising and managing possible TTS. ATAGI also addresses TTS in its weekly statements about COVID-19 vaccines.

Medical officers at the TGA continue to investigate suspected cases and share information with other medicines regulators. 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 Medicines and Health products Regulatory Agency. This allows for rapid assessment and facilitates comparison with the assessments of other medicines regulators. These criteria include:

• evidence of a thrombosis (blood clot)

• thrombocytopenia (blood platelet count below a certain threshold)

• results of blood tests for a specific protein produced by the body to break down clots (D-dimer) and antibodies that activate platelets (anti-PF4 antibodies).

Cases that meet all of the criteria are considered to be confirmed. Cases meeting most of the criteria are classified as probable. Internationally and in Australia, research continues on the utility of other blood tests (including platelet functional assays) on suspected cases to contribute to understanding of TTS. At present, the results of these other tests are not routinely included in our assessment of criteria.

Since last week’s report, a further seven reports of blood clots and low blood platelets have been assessed as TTS and considered likely to be linked to the AstraZeneca vaccine. Three of these cases are considered confirmed, and include a 75-year-old man from Victoria, a 75-year-old man from Western Australia, and a 59-year-old Queensland man who was diagnosed in Victoria. Of these, only the Victorian man remains in hospital, but is responding to treatment and is in a stable condition. The other two patients are not currently in hospital and are thought to be well. Four other newly reported cases are considered to be probable TTS. This includes three men from Victoria aged 65, 70 and 81 years, and a 70-year-old man from NSW. All but one of the newly reported cases was vaccinated after the 8 April 2021 recommendation by ATAGI that Comirnaty is preferred over the AstraZeneca vaccine in adults aged under 50 years.

This takes the total Australian reports of cases assessed as TTS following the AstraZeneca vaccine to 18. So far about 1.8 million doses of the AstraZeneca vaccine have been administered.

With regard to all confirmed cases of TTS mentioned in previous weekly reports, all patients (with the exception of the fatality of a 48-year-old woman from NSW) are recovering and stable.

The reporting rates of TTS in Australia are consistent with what is being seen internationally. However, we believe that a higher proportion of less severe cases may be being reported in Australia. This may be due to high levels of awareness in the community and among the medical profession around TTS along with less strain on the healthcare system around COVID infections with much lower infection rates than internationally.

To 12 May the TGA has received 6 reports of Guillain-Barre Syndrome (GBS) following the 1.8 million administrations of AstraZeneca COVID-19 vaccine. GBS is a rare immune system disorder that causes nerve inflammation and can result in pain, numbness, muscle weakness and difficulty walking. In many cases it resolves within months or a year or two.

In adults, this condition occurs in the population at a background rate of 2 – 3 cases per 100,000 per year. A link between GBS and the AstraZeneca COVID-19 vaccine has not been confirmed either in Australia or internationally so it is unclear whether this is simply a chance association.

However, GBS is considered an adverse event of special interest and is being closely monitored by medicine regulators. The European Medicines Agency safety committee is assessing reports of GBS with the AstraZeneca vaccine as part of the review of all safety data that is regularly submitted by the sponsor. The TGA will continue to monitor and investigate Australian reports, assess worldwide safety data submitted by the sponsor and share information with international counterparts.

Up-to-date information about the expected side effects of the AstraZeneca COVID-19 vaccine can be found in the Consumer Medicine Information (for consumers) and Product Information (for health professionals).

Our monitoring shows that anaphylaxis remains a rare adverse reaction to Comirnaty. 

Our analysis of reports to 2 May shows that there are approximately 20 cases of anaphylaxis reported in Australia per million doses of the Comirnaty vaccine. When cases are reviewed against the internationally accepted Brighton Collaboration definition of anaphylaxis, many of the suspected anaphylaxis reports where adrenaline was given are found to be milder allergic reactions or a sudden stress-related response to vaccination. There have been no deaths reported from anaphylaxis after the Comirnaty vaccine. Reporting of suspected anaphylaxis following vaccination with Comirnaty continues to decline.

Anaphylaxis is a rare adverse event that can occur with any vaccine. People giving vaccines must have medical treatment for anaphylaxis on hand, and must keep people under observation for 15 minutes so that anaphylaxis can be managed, if it happens.

Up-to-date information about Pfizer Comirnaty can be found in the Consumer Medicine Information (for consumers) and Product Information (for health professionals).