EXPERT REACTION: AstraZeneca vaccine linked to bleeding disorder, Pfizer not

Media release

A study whose lead author is a Te Herenga Waka—Victoria University of Wellington epidemiologist has found the Pfizer-BioNTech COVID-19 vaccine used in New Zealand is not associated with a condition that affects the blood known as idiopathic thrombocytopenic purpura (ITP) and other clotting and bleeding events.

ITP is characterised by low counts of platelets, the blood cells that help prevent blood loss when vessels are damaged. A low number of them can result in no symptoms or can lead to an increased risk of bleeding or in some cases clotting.

Professor Colin Simpson, associate dean—research and innovation in the University’s Te Wāhanga Tātai Hauora—Wellington Faculty of Health, was part of the study of 5.4 million people in Scotland, of whom 2.5m had received their first vaccine dose, with colleagues at the University of Edinburgh, where he was reader and director of innovation at the Usher Institute in the College of Medicine and Veterinary Medicine before joining Te Herenga Waka in 2017

The study is the first analysis of ITP, bleeding and clotting events for an entire country following vaccination.

Its findings are good news for New Zealand’s ongoing vaccine rollout, says Professor Simpson.

“It is encouraging we did not identify any increased risk of ITP, bleeding and clotting events in those receiving the Pfizer-BioNTech vaccine,” he says.

ITP may be associated with the Oxford-AstraZeneca vaccine in rare cases, the study suggests.

The very small increased risk of the condition is estimated to be 11 per million doses. The study team says the increased risk of developing ITP after receiving the Oxford-AstraZeneca vaccine remains very substantially smaller than that of developing it because of COVID-19. It says recipients of the vaccine should be made aware of the slight increased risk but it should not deter the rollout of the vaccine.

The study did not include vaccines other than Pfizer-BioNTech and Oxford-AstraZeneca.

Professor Simpson says the team will continue to monitor the safety of the two vaccines as they are rolled out in Scotland.

“We are now planning to update our analysis as the vaccine programme is being extended to younger, healthier individuals and as new vaccines are becoming available,” he says.

“This and other important international work undertaken by researchers at Te Herenga Waka—Victoria University of Wellington is providing the best scientific evidence for policymakers in Aotearoa and other countries.”

The researchers were unable to establish a definitive link between other forms of clotting— including the rare one called cerebral venous sinus thrombosis or CVST—due to the very low number of cases in vaccinated people included in the study.

Those at most risk from ITP tended to be older—a median age of 69 years—and had at least one underlying chronic health condition, such as coronary heart disease, diabetes or chronic kidney disease.

The team, led by the University of Edinburgh, analysed a dataset as part of the EAVE II project, which uses anonymised linked patient data to track the pandemic and the vaccine rollout in Scotland in real time.

It investigated data up to 14 April 2021 for people in Scotland who had received the first dose of either vaccine. By this date, more than 1.7 million had an Oxford-AstraZeneca jab and some 800,000 had a dose of the Pfizer-BioNTech vaccine.

The team—working in collaboration with the Universities of Strathclyde, Aberdeen, Glasgow, Oxford, Swansea and St Andrew’s, Queen’s University, Belfast, and Public Health Scotland—also looked at health records dating back to September 2019 to investigate any previous ITP, clotting or bleeding events.

The data—including GP records on vaccination, hospital admissions, death registrations and laboratory test results—were then compared with those who were yet to be vaccinated to determine if any clotting events were outside what would have been expected pre-pandemic.

The data indicated there was a slight increase in ITP in the second week following vaccination for those who received the Oxford-AstraZeneca vaccine and possibly also increased risk of arterial clotting and bleeding events.

The 11 cases of ITP per million vaccine doses is similar to numbers seen for Hepatitis B, MMR and flu vaccines, which range from 10 to 30 cases of ITP per million doses.

The team found no adverse events in relation to ITP, clotting or bleeding in their analysis for the Pfizer-BioNTech vaccine.

The researchers say a two-week lag for hospital data may mean some data are missing, which possibly limits the study’s findings. Further, the study included relatively few young vaccinated people under 40, especially for the Oxford-AstraZeneca vaccine, as the Scottish vaccination programme followed the recommendations of the UK Joint Committee on Vaccination and Immunisation, which prioritised vaccinations for older and vulnerable adults.

The results are published in the journal Nature Medicine. The study was funded by the Medical Research Council, UK Research and Innovation Industrial Strategy Challenge Fund and Health Data Research UK (HDR UK), and was supported by the Scottish Government.

Additional support was provided through the Scottish Government Director-General Health and Social Care, and the UKRI COVID-19 National Core Studies Data and Connectivity programme led by HDR UK.

Professor Aziz Sheikh, director of the University of Edinburgh’s Usher Institute and EAVE-II study lead, says: “This careful analysis of an entire country’s vaccination programme, which involved the study of over 2.5m first dose vaccines, has found a small increase in the risk of ITP, clotting and bleeding events following the Oxford-AstraZeneca vaccine. This very small risk is important, but needs to be seen within the context of the very clear benefits of the vaccines and potentially higher risks of these outcomes in those who develop COVID-19.”

Professor Chris Robertson from Strathclyde University and Public Health Scotland, says: “This study shows the advantage of being able to link together large national data sets to provide near real time information of vaccine safety, using a number of analytical methods. An important next step is to replicate this work in other settings to ensure the findings are robust.”