First dose ChAdOx1 and BNT162b2 COVID-19 vaccinations and cerebral venous sinus thrombosis

February 2022: Research published in PLOS Medicine uses data from England, Scotland and Wales to look at possible links between having a rare type of blood clot in the brain, and a first dose of two types of COVID-19 vaccine.

Summary in Plain English

The Pfizer-BioNTech (BNT162b2 mRNA) and Oxford-AstraZeneca (ChAdOx1 nCoV-19) vaccines have been found to be safe and effective for use against COVID-19. This includes clinical trials before approval, and studies of national populations throughout the vaccine roll-out.

Vaccines can have side effects, known as adverse events. Most, like headaches and soreness at the injection site, are common and mild. Other events are very rare.

From March 2021, there have been reports in Europe and the United States of a possible link between COVID-19 vaccines – the Oxford-AstraZeneca vaccine in particular – and a rare blood clot in the brain.

This type of clot, called cerebral venous sinus thrombosis (CVST), is a very rare form of stroke.

Why did we carry out this research?

In previous research using data from the Scottish population, we studied possible links between having a first dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccine, and bleeding or blood clot-related side effects.

View the summary of our previous research in this area

This research provides important evidence about the benefits and risks of COVID-19 vaccines for policy makers, healthcare staff and the public.

However, there were not enough people in Scotland who experienced CVST after a first dose for us to analyse the data in a reliable way for this condition. To make the results more accurate, we looked at cases of CVST after the first dose of a vaccine in England and Wales, as well as Scotland.

What data did we use?

The health data we used for this study is stored securely and separately in England, Scotland and Wales, in Trusted Research Environments (TREs). It is made up of data from people’s GP (primary care), hospital (secondary care) and death records, as well as information on COVID-19 testing and vaccine status.

Find out more about Trusted Research Environments

Our study group included all people aged 16 or older who experienced CVST after the first dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccine. The data was analysed for CVST cases between 8 December 2020 and 30 June 2021.

To calculate the risk of experiencing CVST after a vaccine, we compared each person in the 0-28 days after having a vaccine, with a 90-day reference period that ended two weeks before they were vaccinated.

This self-comparison helps to account for a person’s individual risk of CVST, which is influenced by factors like age, genetics, lifestyle and health history.

What did we find?

By pooling the number of CVST cases across the Scotland, England and Wales, we could create a big enough dataset to analyse possible links with vaccination, without sharing individual data between countries.

Oxford-AstraZeneca vaccine

Approximately 4.95 million people in the cohort had a first dose of the Oxford-AstraZeneca vaccine in the period studied. Amongst these, we found:

• There were 45 first cases of CVST in the 90 day reference period
• There were 27 first cases of CVST in the 28 day period after vaccination.

This translates to a risk of CVST approximately twice as high in the 28 days after a vaccine as before vaccination.

There are 3-4 cases of CVST per million people a year in the general UK population. Our analysis suggests approximately one extra case of CVST per four million people in the 28 days after having a first dose of the AstraZeneca vaccine.

Pfizer-BioNTech vaccine

Around 3.18 million people in the cohort had the Pfizer-BioNTech vaccine as a first dose. Amongst these, we found:

• There were 29 cases of CVST in the 90 day reference period
• There were 7 cases of CVST in the 28 day period after vaccination.

We did not find any evidence of people having increased CVST risk in the 28 days after a first dose of the Pfizer-BioNTech vaccine.

What does this mean?

In this study, we found a slightly increased risk of cerebral venous sinus thrombosis (CVST) for people who had a first dose of the Oxford-AstraZeneca vaccine.

We did not find any link between CVST and the Pfizer-BioNTech vaccine.

We used pooled data from Scotland, England and Wales to study this rare side effect in a larger cohort. This method for safely gathering large datasets across nations will allow us to look at other rare side effects in the future, for COVID-19 and other diseases. 

Read an Interview with lead author, Dr Steven Kerr, on PLOS blogs

Note

This plain English summary was created with the support and feedback of the EAVE II Public Advisory Group (PAG). This summary in particular was reviewed by Farzana K and Carrol L.

To learn more about the PAG, see: Our EAVE II Public Advisory Group (PAG) | The University of Edinburgh

DaC-VaP

This paper is from the EAVE II connected project, National Core Study - Data and Connectivity: COVID-19 Vaccines Pharmacoviligance (DaC-VaP)

Read more on the DaC-VaP project research page

Find out more about EAVE II's connected projects