Comparative efficacy of ChAdOx1 and BNT162b2 COVID-19 vaccines

In a recent study published on the Preprint Server medRxiv*, researchers compared the effectiveness of Pfizer-BioNTech BNT162b2 and Oxford-AstraZeneca ChAdOx1 Coronavirus Disease 2019 (COVID-19) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and COVID disease -19 in the fields of health and nursing social services in England. Overall, no substantial difference in results was observed up to 20 weeks after vaccination in either case, suggesting strong vaccine-induced immunity against COVID-19.

To study: Comparative efficacy of ChAdOx1 versus BNT162b2 COVID-19 vaccines in healthcare and social service workers in England: a cohort study using OpenSAFELY. Image Credit: Viacheslav Lopatin /


The Pfizer-BioNTech BNT162b2 COVID-19 messenger ribonucleic acid (mRNA) vaccine and the Oxford-AstraZeneca ChAdOx1 nCoV-19 vaccine have been approved and widely administered as part of the UK’s National Immunization Program. These vaccines have been approved for use following Phase III randomized controlled trials (RCTs) which have demonstrated clear evidence of vaccine efficacy.

In vaccine deployment settings, observational studies have confirmed the efficacy of the vaccine. However, to date, no RCT has directly compared the BNT162b2 and ChAdOx1 vaccines to estimate their relative efficacy against SARS-CoV-2 infection and disease in the same population.

Since target populations and parameters differ significantly from clinical trials performed prior to vaccine authorization, researchers undertook the present study to mimic such a comparative vaccine efficacy trial using observational data. . Such comparisons in observational data are difficult without administration of both vaccines to the same population at the same time.

Health and social service (HCW) workers were among the first group of people eligible for vaccination due to their high occupational risk of exposure to SARS-CoV-2, which causes COVID-19. As HCW status becomes a major confounding factor in studying the effect of vaccination on COVID-19 outcomes, researchers chose to study this group in isolation, thereby eliminating the confounding factor. .

By eliminating possible confounding and selection bias, as seen in comparisons of vaccinated and unvaccinated groups, the present study is the first to assess the efficacy of BNT162b2 and ChAdOx1 vaccines in a direct comparison in an experimental or experimental setting. observational.

About the study

The study cohort included 317,341 healthcare workers vaccinated between January 4 and February 28, 2021, who were registered with a general practice using the TPP SystmOne clinical information system in England. During the study period, both vaccines were widely used. Of the study cohort, 79.8% were vaccinated with BNT162b2 and 20.2% with ChAdOx1.

The vaccinated healthcare workers included in the study were between 18 and 64 years old. These workers were not clinically extremely vulnerable.

Without any way to anticipate what type of vaccine to associate with workers’ health status, healthcare workers were vaccinated with BNT162b2 or ChAdOx1 administered as part of the national COVID-19 vaccine rollout.

Researchers used a target trial design to assess the efficacy of ChAdOx1 versus BNT162b2 in healthcare workers, including the efficacy of the second dose using the primary care database linked to OpenSAFELY- TPP. This database covers 24 million people registered in general medicine, or about 40% of the English population.

The researchers defined three outcomes occurring within 20 weeks of vaccination in this study. These results included a positive SARS-CoV-2 test, participation in COVID-19 accidents and emergencies (A&E), and an unplanned COVID-19 hospital admission.

Serious illness, such as ICU or intensive care admission, and mortality events were minimal; therefore, the researchers did not fully investigate these findings.

The results of the study

The researchers presented the participants vaccinated with each vaccine by date. The graph showed that BNT162b2 was on average administered earlier than ChAdOx1.

For comparative effectiveness, the researchers observed that after vaccination, the absolute risk difference ChAdOx1 versus BNT162b2 per 1,000 people for a positive SARS-CoV-2 test, COVID-19 A&E presences and COVID-19 hospitalizations. They found that the results were similar for those who received the BNT162b2 and ChAdOx1 COVID-19 vaccines.

For each outcome based on the fully fitted model, the marginal cumulative impact for ChAdOx1 and BNT162b2, their difference and the risk ratio are shown. Models that assumed piecewise constant risks yielded similar effect estimates (Supplementary Figure S2). Models with a less extensive confounding adjustment gave very similar estimates (Supplementary Figure S1), suggesting that recipients of each vaccine were similar after accounting for differences in vaccine allocation over space and time ( like all models).

At 20 weeks, researchers found a small advantage for the BNT162b2 vaccine over ChAdOx1 in terms of immune people who tested positive for SARS-CoV-2. However, the fact that recipients of BNT162b2 were more likely to receive their second dose earlier conferred a stronger protective effect which was not considered in this study. The difference in COVID-19 hospital attendance and admission events for the two vaccines was much less than 1 event per 1,000 people in either direction.

The researchers cautioned that the results of the present study, which consisted primarily of healthy healthcare workers, may not reflect comparative effectiveness in more vulnerable groups such as the elderly or the immunocompromised. Additionally, the dominant variant circulating during this study was the Alpha variant, while the current variant is the Delta variant, against which the vaccine’s efficacy may vary. In particular, the researchers noted that the potential immunological decline may differ.


Direct comparisons of first and second dose effectiveness between COVID-19 ChAdOx1 and BNT162b2 vaccines showed similar incidence rates for infection and COVID-related hospital attendance and admission -19. This indicates strong protective effects for both vaccines. The researchers commissioned further studies to assess the comparative effectiveness of newer and more common variants and to assess the longer-term effectiveness.

*Important Notice

medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.

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