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Two-dose ChAdOx1 nCoV-19 vaccine protection against COVID-19 hospital admissions and deaths over time in Scotland & Brazil

December 2021: The protection offered by the Oxford-Astra Zeneca Covid-19 vaccine declines after three months of receiving two doses, a study says.

Infographic summarising key findings that protection from COVID-19 vaccine wanes after about 3 months
This image was created by the Usher Institute, The University of Edinburgh to accompany a research study by the EAVE II team funded by the Medical Research Council [MR/R008345/1] and the Data and Connectivity National Core Study [MC_PC_20058], and supported by BREATHE - The Health Data Research Hub for Respiratory Health [MC_PC_19004]. This research was done in partnership with the Fiocruz VigiVac project in Brazil partially funded by the Fazer o Bem Faz Bem programme and supported by the Brazilian National Research Council and FAPERJ [E-26/210.180/2020]. © 2021 Usher Institute, The University of Edinburgh. All rights reserved.

Two-dose ChAdOx1 nCoV-19 vaccine protection against COVID-19 hospital admissions and deaths over time: a retrospective, population-based cohort study in Scotland and Brazil

Katikireddi, V.; Cerqueira-Silva, T.; Vasileiou, E; Robertson, C; et. al.

The Lancet

Published online: December 20, 2021

Available online at: https://doi.org/10.1016/S0140-6736(21)02754-9

Download the infographic

This infographic was created by the Usher Institute's Graphic Designer, Dawn Cattanach

Press Release

The findings – drawn from datasets in two countries – suggest that booster programmes are needed to help maintain protection from severe disease in those vaccinated with Oxford-Astra Zeneca, experts say.

Fivefold increase in risk of hospitalisation or death after five months

Researchers from Scotland and Brazil analysed data for two million people in Scotland and 42 million people in Brazil who had been vaccinated with the Oxford-Astra Zeneca vaccine.

In Scotland, when compared with two weeks after receiving a second dose, there was approximately a fivefold increase in the chance of being hospitalised or dying from Covid-19 nearly five months after being double vaccinated. 

The decline in effectiveness begins to first appear at around three months, when the risk of hospitalisation and death is double that of two weeks after the second dose, experts say.

The risk increases threefold just short of four months after the second vaccine dose. Similar numbers were seen for Brazil.

Researchers were able to compare data between Scotland and Brazil as they had a similar interval between doses – 12 weeks – and initial prioritisation of who was vaccinated – people at highest risk of severe disease and healthcare workers.

Different variants

The dominant variant was different in each country during the study period – Delta in Scotland and Gamma in Brazil – meaning the decline in effectiveness is likely because of vaccine waning and the impact of variants. 

The study, published in The Lancet, also estimated vaccine effectiveness at similar fortnightly intervals by comparing outcomes of people who have been jabbed with those who are unvaccinated.

But experts warned these figures should be treated with caution because it is becoming harder to compare unvaccinated people to vaccinated people with similar characteristics, particularly among older age groups where so many people are now vaccinated.

Vaccines have been a key tool in fighting the pandemic, but waning in their effectiveness has been a concern for a while. By identifying when waning first starts to occur in the Oxford-Astra Zeneca vaccine, it should be possible for governments to design booster programmes that can ensure maximum protection is maintained.

If eligible for a booster and you have not had yet had one, I would highly recommend that you book one soon.

Professor Aziz SheikhDirector of the University of Edinburgh’s Usher Institute and EAVE II study lead

EAVE II study

The study is part of the EAVE II project, which uses anonymised linked patient data in Scotland to track the pandemic and the vaccine roll out in real time.

The research team included scientists from the Universities of Edinburgh, Glasgow, Aberdeen, Strathclyde, and St Andrew’s; Public Health Scotland; Victoria University of Wellington; Fiocruz; Universidade do Estado do Rio de Janeiro and Universidade Federal de Bahia.

Our analyses of national datasets from both Scotland and Brazil suggest that there is considerable waning of effectiveness for the Oxford AstraZeneca vaccine, with protection against severe Covid-19 falling over time.

We studied two million people in Scotland and over 42 million people in Brazil who had received two doses of the Oxford-AstraZeneca vaccine. More than four months after receiving a second dose, the risk of experiencing either a Covid-19 hospitalisation or death was approximately five times greater than the period of maximum vaccine protection after accounting for changes in infection rates and a range of other factors.

Our work highlights the importance of getting boosters, even if you’ve had two doses of the Oxford AstraZeneca vaccine, as soon as you are able to.

Professor Vittal KatikireddiUniversity of Glasgow

The study was funded by the Medical Research Council, UK Research and Innovation Industrial Strategy Challenge Fund, the National Institute for Health Research 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.

The EAVE II - FioCruz Partnership

The EAVE II team has now worked extensively with our colleagues based in the FioCruz Institute in Brazil to conduct COVID-19 anaylses. You can find out more about this partnership and the work that has been accomplished by clicking on the link below.

Watch DaC-VaP-2: Developing International Partnerships