Single Dose of Pfizer or Oxford Vaccine May Significantly Reduce Risk of Elderly COVID-19 Hospitalizations

This article is based on research findings that are yet to be peer-reviewed. Results are therefore regarded as preliminary and should be interpreted as such. Find out about the role of the peer review process in research here. For further information, please contact the cited source.

A new preprint study posted to Preprints with The Lancet suggests that a single dose of either the Pfizer–BioNTech or Oxford University–AstraZeneca vaccine can substantially reduce the risk of COVID-19-related hospitalizations in the elderly.¹

According to the World Health Organization, elderly or frail populations are some of the most vulnerable to severe COVID-19. As a result, many have spent the majority of 2020 and 2021 shielding from SARS-CoV-2, the virus responsible for COVID-19. Whilst the development and approval of vaccines may provide hope for a route out of isolation, there has been less focus in clinical trials on the effectiveness of COVID-19 vaccination in those aged 80 years and above.  The AvonCAP project is addressing this.

How was the interim analysis conducted?

Although the study is ongoing, the research behind this interim analysis was conducted over the course of approximately two months in two major hospitals in Bristol, UK. Participants were identified by the following criteria:

· Individuals aged 80 years or above as of March 31, 2021

· Admitted to either North Bristol or University Hospitals Bristol between December 18, 2020 and February 26, 2021

· Showed signs or symptoms of respiratory illness

As a test-negative case-control study, the resulting cohort was then split into either the “case” or “control” group based upon whether the patient had tested positive or negative for COVID-19. The next step was to determine whether and when participants had received a first dose of either the Pfizer–BioNTech or Oxford University–AstraZeneca vaccine. To reduce observer bias, this information was collected blindly; individuals collecting the data were not aware of participants’ COVID-19 test results.

How does this data become an indicator of vaccine effectiveness? Dr Peter English, consultant in Communicable Disease Control, explained that the researchers were “able to calculate the proportion of the patients in the case and control groups that had been vaccinated at least 14 days previously.”

“Comparing these proportions – the odds of having been vaccinated – gives the 'odds ratio’, from which you can get a good estimate of the protective effect of the vaccines,” he said.

Consistent with existing evidence, but caution is advised and more research needed

The effectiveness of a single dose was found to be 71.4% and 80.4% for the Pfizer–BioNTech and Oxford University–AstraZeneca vaccines, respectively. However, it is here that the authors note an important factor to consider; in the UK, the first administration of the Oxford University–AstraZeneca vaccine occurred nearly one month after that of the Pfizer–BioNTech. The researchers adjusted their effectiveness analysis of the Pfizer–BioNTech vaccine accordingly to include only the time in which it became available post-authorization. Even with this consideration, effectiveness of a single dose of the Pfizer–BioNTech vaccine was still found to be 79.3%. Overall, the results suggest that a single dose of either vaccine can significantly reduce the risk of COVID-19-related hospitalization in those aged 80 years and above.

Dr Michael Head, senior research fellow in Global Health at the University of Southampton, advised a certain level of caution when interpreting the results: “There are limitations, such as the wide confidence intervals around both the Pfizer and Oxford–AstraZeneca data, indicating uncertainty as to the true level of protection given by the vaccine.”

But Head encouraged positivity too: “The results here are underpinned by similar findings from the emerging body of evidence that covers clinical trials and analyses of real-world data.”
“Overall, this looks like further good news around COVID-19 vaccines.”

In the paper's conclusion, the researchers behind the AvonCAP project suggest that the next step will be to explore the effectiveness of the COVID-19 vaccines in reducing other outcomes, such as length of hospital admission and mortality.

Reference: Hyams C, Marlow R, Maseko Z et al. Assessing the effectiveness of BNT162b2 and ChAdOx1nCoV-19 COVID-19 vaccination in prevention of hospitalisations in elderly and frail adults: a single centre test negative case-control study. Preprints with The Lancet. Available at SSRN: doi: 10.2139/ssrn.3796835.