Study suggests rates of thrombocytopenia and VTE increase after Pfizer

Jolyon Attwooll

6/08/2021 4:32:41 PM

The ‘interesting’ research contains a number of clinical takeaways for general practice, but shows no clear link to TTS, a GP says.

Pfizer vaccine vial.
An increased rate of thrombocytopenia was observed after both the first and second doses of the Pfizer vaccine. (Image: AAP)

A study using primary care data from a broad population sample in Spain has suggested there is a higher rate of thrombocytopenia among those receiving the Pfizer vaccine.
The study authors report similar profiles for both vaccines with a safety signal observed for venous thromboembolism (VTE) after one dose of Pfizer and similarly following the first dose of AstraZeneca, although the confidence interval was wider.
There was also an increased rate of thrombocytopenia observed after the first and second doses of the Pfizer vaccine. However, the research also echoed previous studies that found thromboembolism and thrombocytopenia rates are much higher among COVID-19 patients than those receiving vaccines.
Dr Ai-Vee Chua is a NSW GP who worked in a multidisciplinary group that shaped consensus guidelines for doctors to diagnose and respond to suspected cases of thrombosis with thrombocytopenia syndrome (TTS) – also known as vaccine-induced immune thrombotic thrombocytopenia (VITT).
She says one takeaway from the study for GPs is an awareness of the greater risk of VTE and thrombocytopenia following the Pfizer vaccine.
‘While the Pfizer vaccine seems to have developed a reputation in Australia of being the “good” vaccine, patients need to be made aware that Pfizer vaccination also carries with it a rare risk of serious side effects,’ Dr Chua told newsGP. ‘Venous thromboembolism and thrombocytopenia are possibilities in addition to what we already know in relation to myocarditis or pericarditis risk.
‘We should have a low threshold for investigating these patients should they present with symptoms suggestive of a deep vein thrombosis [DVT], pulmonary embolism or low platelets.’
Dr Chua notes that thromboembolic events were identified using primary care records and that the study did not include details on hospital diagnoses.
‘Therefore, events more likely diagnosed at the hospital level, such as cerebral venous sinus thrombosis, were not captured and the incidence of thromboembolic events may have been underestimated across all cohorts,’ she said.
However, Dr Chua says the study provides no indication of any link between Pfizer and the rare blood-clotting condition associated with AstraZeneca.
The analysis, which has not been peer-reviewed, tracked the prevalence of VTE, arterial thromboembolism (ATE), thrombocytopenia, and TTS, then compared rates among people vaccinated with the Pfizer and AstraZeneca vaccines to the wider population – both before the pandemic and among those who had COVID-19.
There were 1,372,213 people who received their first vaccine dose (69% Pfizer, 31% AstraZeneca) considered in the study. Of those, 778,534 were also observed following a second Pfizer dose.
Findings were compared to 222,710 people with a COVID-19 diagnosis and 4,570,149 individuals in the wider population cohort.
The rate of VTE increased eight-fold after diagnosis of COVID-19, the study suggests. It reported no safety signals for ATE or TTS among those taking either vaccine, but said further research is needed ‘to investigate the causality in the observed associations’.
Professor Paul Monagle, a paediatric haematologist at the Royal Children’s Hospital in Melbourne, said it was unsurprising that no notable distinctions had emerged when compared to the broader population.
‘It’s difficult when you just compare all VTE because there’s so much noise,’ he told newsGP.
‘The background rate of VTE in the community is just so high that it’s very hard to distinguish a statistically significant difference, I would have thought.
‘If you think that the rate of TTS following adenovirus vector vaccines is maybe one in 30,000 or one in 40,000, and yet you have got a rate of VTE in the community that’s much higher than that, [then] your ability to pick up events with this kind of study is really difficult.’
On Thursday, Australia’s Therapeutic Goods Administration (TGA) announced the sixth death associated with TTS following AstraZeneca vaccination in its latest weekly safety report. The TGA has identified 93 potential cases of TTS (57 confirmed, 36 probable) from more than 6.8 million AstraZeneca doses administered in Australia so far.
The authors of the Spanish study also emphasised the low risks of vaccination compared to COVID-19.
‘For each of the approved vaccines, rates of thrombosis can be expected to be far lower than rates among those persons with COVID-19, a disease which the vaccines are so effective at preventing,’ they write in the study.
Professor Monagle says those findings make sense.
‘There’s no doubt that COVID-19 is a very thrombogenic condition, so it is not surprising that COVID-19 itself gives a signal for much higher rates of thrombosis than for background rates or against any of the vaccines,’ he said.  
‘If you are sick and hospitalised with COVID, your VTE rate is high, which is why I think there has been such a push on working out the best thromboprophylaxis strategies for those patients.’
Another important consideration related to the interpretation of the study, Dr Chua says, is that the cohort who received Pfizer were mostly aged over 70, while almost all receiving AstraZeneca were aged under 70.
‘[The people who received AstraZeneca] were therefore younger and healthier than the Pfizer cohort, as well as the general population,’ Dr Chua said.
‘This is in contrast to our situation in Australia, whereby the Pfizer vaccine has largely been prioritised for the younger population and AstraZeneca has been utilised predominantly in the older population.’
However, like Professor Monagle, she stressed the increased risks associated with COVID-19 compared to vaccination.
‘Patients need to be made aware that while there are possible small risks with either of the currently available COVID-19 vaccines, the risk of thrombosis, thrombocytopenia, myocarditis or pericarditis, chronic illness [long COVID], hospitalisation and death are very much higher with COVID-19 infection itself,’ Dr Chua said.
‘We play an important role in helping our patients understand this balance of risk in their decision about COVID-19 vaccination for themselves and their families.’
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