UK public health authorities review early data on the effectiveness of Pfizer and AstraZeneca COVID-19 vaccines

A team of scientists in the UK recently evaluated the effectiveness of the Pfizer (BNT162b2) and AstraZeneca (ChAdOx1) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The findings reveal that both vaccines are effective in significantly reducing SARS-CoV-2 infection in older adults and providing long-term protection. The study is currently available on the medRxiv* preprint server.


The recent outbreak of coronavirus disease 2019 (COVID-19) has caused a dramatic loss of human life and devastated the healthcare systems and socioeconomic structures of many countries globally. As an attempt to control the viral spread, several vaccines and therapeutics have been developed in record time. Some of these vaccines have also received emergency use approval from various national regulatory bodies authorities. In this context, the UK became the first country to roll out an mRNA-based COVID-19 vaccine developed by Pfizer and BioNTech (BNT162b2). Afterward, an adenovirus-based vaccine developed by the AstraZeneca (ChAdOx1) was also included in the UK’s COVID-19 vaccination program.

The interim phase 3 clinical trial data indicate that both Pfizer and AstraZeneca vaccines are highly effective against SARS-CoV-2 when administered twice at intervals of 3 weeks and 4 weeks, respectively. However, there is evidence suggesting that the level of protection can be improved by increasing the dosing interval. Based on these findings, the UK Joint Committee on Vaccination and Immunization has recommended increasing the dosing interval up to 12 weeks for both vaccines.

The current study was conducted to analyze the impacts of both Pfizer and AstraZeneca vaccines on confirmed COVID-19 patients aged 70 years and above. The efficacy of the vaccines in reducing emergency hospitalization and mortality has also been analyzed. Because the newly emerged B.1.1.7 variant of SARS-CoV-2 has become the dominant strain in the UK, the vaccine effectiveness in managing this significantly more infectious strain has been analyzed in the current study.

Important observations

The efficacy of the two-dose regimen of the Pfizer vaccine and a single dose of the AstraZeneca vaccine was analyzed in the study.

The study findings revealed that a single dose of the Pfizer vaccine is 60 – 70% effective in reducing symptomatic SARS-CoV-2 infection among the older population in England; whereas, a two-dose vaccination regimen was found to be 85 – 90% efficacious. Regarding the AstraZeneca vaccine, the findings revealed that a single vaccine dose is approximately 60 – 75% effective in reducing symptomatic infection. With the Pfizer vaccine, an effectiveness of 70% was observed after 28 days of first dose administration and an effectiveness of 90% was observed after 14 days of second dose administration. With the AstraZeneca vaccine, effectiveness of 60% was observed after 28 days of vaccination, which further increased to 75% after 35 days of vaccination.  

The scientists estimated that compared to unvaccinated individuals, BNT162b2-vaccinated individuals who later on acquired SARS-CoV-2 infection had 44% and 51% lower risks of hospitalization and death, respectively. The AstraZeneca vaccine ChAdOx1 also showed some level of protection (37%) against hospitalization.     

During the initial phase of the roll-out, older people aged 80 years and above were the target population for vaccination. The scientists noticed that people vaccinated with the Pfizer vaccine during this phase had a higher rate of testing SARS-CoV-2 positive within a few days after vaccination. This indicates that the risk of COVID-19 is higher in the initially targeted population and that they may acquire infection before developing desired immune responses to the vaccine. The vaccine started providing protection after 10 – 13 days of immunization, which reached 70% after 28 – 34 days and remained constant.

By analyzing the spike-gene target failure (SGTF) status, the scientists revealed that the B.1.1.7 variant of SARS-CoV-2 is currently predominating in England and that the variant has negligible influence on the vaccine effectiveness.

Study significance

The study findings collectively reveal that both Pfizer and AstraZeneca vaccines are effective in preventing SARS-CoV-2 infection and reducing COVID-19-related hospitalization. In addition, the Pfizer vaccine has shown an added benefit of reducing COVID-19-related mortality. A clear vaccine effect has been observed with a single dose, highlighting the option of maximizing the number of vaccinated people with a single dose to better manage the pandemic. Importantly, a significant proportion of vaccinated people have been found to develop SARS-CoV-2 infection, particularly during the first few weeks after vaccination. This highlights the importance of maintaining pandemic-related precautions even after vaccination.       

*Important Notice

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

Journal reference:
Dr. Sanchari Sinha Dutta

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.


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