Thrombosis with thrombocytopenia syndrome (TTS) is a rare event involving serious blood clots with a low blood platelet count. Emerging evidence suggests a causal link to 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 the Comirnaty (Pfizer) vaccine be preferred over the AstraZeneca vaccine in people aged under 50. 

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 United Kingdom’s (UK) Medicines and Health products Regulatory Agency. 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. Occasionally, we may not be able to obtain sufficient information to determine whether a case meets the diagnostic criteria. In Australia and internationally, research continues on the utility of other blood tests (including platelet functional assays) that may help in the assessment on suspected cases.

Since last week’s report, a further six reports of blood clots and low blood platelets have been assessed as TTS and considered likely to be linked to the AstraZeneca vaccine. Three are newly reported confirmed cases – one in a 57-year-old woman from Victoria, one in a 53-year-old man from South Australia and the final case in an 18-year-old woman in Queensland. Review of an earlier case in a 79-year-old Victorian man has now been confirmed as meeting the diagnostic criteria. Two other new cases, in an 87-year-old woman from South Australia and a 71-year-old woman from Victoria, were deemed probably related to the vaccine, but more clinical information is required for any confirmations to be made.

The 18-year old was vaccinated prior to the 8 April 2021 recommendation by ATAGI that Comirnaty is preferred over the AstraZeneca vaccine in adults aged under 50 years. The information reported to the TGA meets the criteria for confirmed TTS. However, the case remains under investigation as there are ongoing clinical investigations including consideration of other medical conditions. 

This takes the total Australian reports of cases assessed as TTS following the AstraZeneca vaccine to 24. So far about 2.1 million doses of the AstraZeneca vaccine have been administered. Overall, 21 of these cases are considered confirmed and three are considered probable. Three of the four cases reported as probable in last weeks’ report have since been confirmed as TTS. The other case remains under investigation. 

The confirmed and probable cases have occurred in 15 men and nine women aged 18–87 years old. The median age was 66. The most common types of clots were deep vein thrombosis (12 cases), pulmonary embolism (12 cases) and portal vein thrombosis (4 cases). There were also clots found in a variety of other locations in the body, including two cases with cerebral venous sinus thrombosis. Where the information was available, the most common time to onset or diagnosis was 14 days, with a range of 2–44 days after vaccination. In some cases with a longer time between vaccination and diagnosis, patients had experienced some symptoms at an earlier stage, but there may have been complicating factors that delayed a clear diagnosis.

All confirmed and probable cases of TTS so far (with the exception of the fatality of a 48-year-old woman from NSW) are recovering and stable. The information currently available to the TGA indicates that 21 of the patients are out of hospital, while the two that remain in hospital are responding to treatment and stable.

The reporting rates of TTS in Australia remain consistent with what is being seen internationally, including in Europe, the UK, the Middle East and Canada.

Guillain-Barre Syndrome (GBS) is considered an adverse event of special interest and is being closely monitored by international medicine regulators.

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, it occurs in the population at a background rate of 2 – 3 cases per 100,000 per year.

A possible link between GBS and the AstraZeneca COVID-19 vaccine remains under investigation in Australia and internationally. As of 19 May 2021, we have received eight reports of GBS in patients vaccinated with the AstraZeneca COVID-19 vaccine. The TGA continues to monitor and investigate Australian adverse event reports for GBS.

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).

Diarrhoea and vomiting can occur following vaccination with the Comirnaty vaccine. These adverse events were recently added to the Product Information for Comirnaty based on post-market adverse event data.

Vomiting and diarrhoea have been commonly reported in Australia after vaccination with Comirnaty. To 16 May 2021, the TGA has received 281 reports of diarrhoea and 335 reports of vomiting following vaccination with the Comirnaty vaccine. Of those who reported duration, most said they were better after two days. We have also received around 1000 reports listing nausea as a suspected side effect.

These are expected side effects from this vaccine, and are reasonably common side effects for many medicines and vaccines more broadly. Clinical trials of the Comirnaty vaccine reported just under 3% of people experienced gastrointestinal side effects such as nausea, vomiting, diarrhoea and abdominal pain.

We are aware of cases reported overseas of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the membrane around the heart) following vaccination with Comirnaty. There is no indication at the moment that these cases are due to the vaccine. However, the TGA is monitoring and investigating Australian reports of these adverse events.