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Pfizer-BioNTech and AstraZeneca vaccines offer high protection against severe COVID-19, six months after second doses, study finds

A health care worker preparing a COVID-19 vaccine dose

Press release issued: 20 July 2022

Protection against severe COVID-19 by two doses of Pfizer-BioNTech and AstraZeneca COVID-19 vaccines remained high up to six months after second doses, finds new research which analysed NHS health record data on over seven million adults. Reassuringly, the University of Bristol-led study published in The BMJ today [July 20], found protection in older adults aged over 65 years, and in clinically vulnerable adults.

Researchers from Bristol Medical School, Oxford University and the London School of Hygiene and Tropical Medicine sought to investigate how quickly vaccine effectiveness waned over time in adults without prior SARS-CoV-2 infection and who received two doses of BNT162b2 (Pfizer-BioNTech) or ChAdOx1 (AstraZeneca) COVID-19 vaccine compared with unvaccinated individuals.

Using linked GP, hospital, and COVID-19 records on 1,951,866 and 3,219,349 adults who had received two doses of BNT162b2 and ChAdOx1, respectively and 2,422,980 unvaccinated adults, researchers were able to provide a clearer picture of vaccine effectiveness against COVID-19 hospital admission, COVID-19 death, and positive SARS-CoV-2 test.

Rates of COVID-19 hospital admission and COVID-19 death were substantially lower among vaccinated than unvaccinated adults up to six months after their second dose.  Vaccine effectiveness against these events was found to be at least 80 per cent for BNT162b2, and at least 75 per cent for ChAdOx1. However, waning vaccine effectiveness against infection with SARS-CoV-2 meant that rates in vaccinated individuals were similar to or higher than in unvaccinated individuals by six months after the second dose.

Dr Elsie Horne, Senior Research Associate in Medical Statistics and Health Data Science in Bristol Medical School: Population Health Sciences (PHS) and the study's lead author, said: "Until now there has been limited and conflicting evidence relating to the rate of waning following second dose of COVID-19 vaccines, whether it extends to severe COVID-19, and whether the rate differs according to age and clinical vulnerability.

"Although we found that protection against severe COVID-19 provided by two doses of vaccine wanes over time, the very high initial protection means that, despite waning, protection remains high six months after the second dose. This finding was consistent across all adults, including older adults and those who are at risk of severe COVID-19."

Jonathan Sterne, Professor of Medical Statistics and Epidemiology in Bristol Medical School: PHS, Director of the National Institute for Health and Care Research Bristol Biomedical Research Centre (NIHR Bristol BRC), Director of Health Data Research UK (HDR UK) South-West and co-author, added: "We found that the rate at which vaccine effectiveness waned was consistent across subgroups defined by age and clinical vulnerability. Studying how long COVID-19 vaccines remain effective continues to be important to scheduling and targeting of booster vaccinations."

The researchers now plan to lead a follow-up study looking at vaccine effectiveness to one year post-second dose and into the era of the Omicron variant. They are also investigating the rate of waning in vulnerable clinical subgroups, such as those with chronic kidney disease and with cancer.

The study was supported by two COVID-19 National Core Studies (NCS) programmes: COVID-19 Longitudinal Health and Wellbeing and COVID-19 Data and Connectivity; Asthma UK; NIHR (National Institute for Health and Care Research) and Wellcome. TPP provided technical expertise and infrastructure pro bono in the context of a national emergency.


'Waning effectiveness of BNT162b2 and ChAdOx1 covid-19 vaccines over six months since second dose: OpenSAFELY cohort study using linked electronic health records' by E Horne et al in The BMJ [open access]

Further information

About the National Institute for Health and Care Research (NIHR)
The mission of the National Institute for Health and Care Research (NIHR) is to improve the health and wealth of the nation through research. We do this by:

  • Funding high quality, timely research that benefits the NHS, public health and social care;
  • Investing in world-class expertise, facilities and a skilled delivery workforce to translate discoveries into improved treatments and services;
  • Partnering with patients, service users, carers and communities, improving the relevance, quality and impact of our research;
  • Attracting, training and supporting the best researchers to tackle complex health and social care challenges;
  • Collaborating with other public funders, charities and industry to help shape a cohesive and globally competitive research system;
  • Funding applied global health research and training to meet the needs of the poorest people in low and middle income countries.

NIHR is funded by the Department of Health and Social Care. Its work in low and middle income countries is principally funded through UK Aid from the UK government.

About the National Institute for Health and Care Research Bristol Biomedical Research Centre (NIHR Bristol BRC)
The National Institute for Health and Care Research Bristol Biomedical Research Centre’s (NIHR Bristol BRC) innovative biomedical research takes science from the laboratory bench or computer and develops it into new drugs, treatments or health advice. Its world-leading scientists work on many aspects of health, from the role played by individual genes and proteins to analysing large collections of data on hundreds of thousands of people. Bristol BRC is unique among the NIHR’s 20 BRCs across England, thanks to its expertise in ground-breaking population health research.

About OpenSAFELY
OpenSAFELY is a new and highly secure software platform built during the COVID-19 pandemic for safe analysis of electronic health records data, covering 57.9 million patients in England.  It was specifically built to address concerns around patient privacy when doing large scale research with patient records. Analysts write code in OpenSAFELY to answer important research questions without needing direct access to the underlying patient records; all platform activity is publicly logged for transparency; all code is shared automatically for other researchers to review and efficiently re-use. This secure and transparent approach to using medical data has received strong support from privacy advocates, including in parliament, and from an independent Citizens’ Jury on data sharing during COVID-19 commissioned by NHSx and the National Data Guardian. OpenSAFELY is a collaboration between the Bennett Institute at the University of Oxford, electronic health record software suppliers TPP and EMIS, and academic teams at LSHTM and Bristol, with NHS England acting as data controller.

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