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First dose of Pfizer-BioNTech or Oxford-AstraZeneca vaccines has sturdy protecting impact in opposition to COVID-19 in older adults

To help contain the 2019 coronavirus disease (COVID-19) pandemic, the UK launched its vaccination campaign on December 8, 2020. To date, more than 77% of the UK adult population have had at least one dose of the COVID-19 vaccine.

Study: Estimating First Dose Efficacy of COVID-19 Vaccine Against Mortality in England: A Quasi-Experimental Study. Image Source: Melinda Nagy / Shutterstock.com

background

Several COVID-19 vaccines have received Emergency Authorization (EUA) from the UK Medicines and Health Products Regulatory Authority (MHRA), which has allowed many researchers to evaluate the effectiveness of these vaccines in real-life conditions. For example, the two messenger ribonucleic acid vaccines (mRNA) developed by Pfizer-BioNTech (BNT162b2) and Moderna (mRNA-1273) showed an efficacy rate of 95%, while the Oxford-AstraZeneca vaccine (ChAdOx1) showed a lower efficacy of about 70%.

Assessing the effectiveness of these vaccines is essential to determine the impact of the vaccination program. Previous studies have estimated the effectiveness of vaccines by comparing the number of hospitalizations, susceptibility to infection, and mortality rates between vaccinated and unvaccinated people. In these studies, however, certain confounding factors and changes over time that influenced the calculations of the effectiveness of the vaccine were often not taken into account, which subsequently led to a biased overestimation of the effectiveness of the vaccine.

RDD to evaluate the effectiveness of the COVID-19 vaccine

Many researchers believe that under certain conditions, such as eligibility to receive a vaccine based on a continuous variable, regression discontinuity design (RDD) can be used to obtain impartial estimates of vaccine effectiveness. In fact, RDD can be used even in the presence of unmeasured confounders.

For example, RDD approaches have recently been used to show that the Bacillus Calmette-Guérin (BCG) vaccine has no effects on COVID-19 disease. More specifically, the RDD approach helped determine that the relationship between BCG vaccination rates and reductions in COVID-19 cases in different countries was the result of unmeasured confounders.

In summary, the researchers believe the RDD approach can be used to accurately calculate the effectiveness of the COVID-19 vaccine as many countries around the world have implemented an age-based vaccine rollout strategy.

Assessment of vaccine effectiveness

A new study published on the preprint server medRxiv * estimates the effectiveness of the vaccine against COVID-19 mortality in England using the RDD approach. In their work, a fuzzy RDD method was used to assess the impact of vaccination on the risk of death from COVID-19.

Here, deaths from COVID-19 disease were compared to deaths from non-COVID-19 factors in people between the ages of 75-79 and 80-84 years. The RDD took advantage of the discontinuity in vaccination rates created by the UK strategy of age-based vaccination of priority groups.

Study results

The researchers found a reduction in death rates from COVID-19 in people over the age of 80. This finding reassured the scientists that the decrease in the death rate in the older age group was mainly due to the availability of vaccines rather than residual confounding factors.

Using suitability as a criterion for vaccination, the RDD approach used herein also indicated seropositivity in subjects who received a single dose of the vaccine. As a result, a decrease in the COVID-19 death rate has been observed. Remarkably, a similar trend was not observed in the unvaccinated group.

The researchers estimated the effectiveness of the vaccine at 70.5% for the vaccinated group, who were at least 80 years old and seropositive. In addition, the scientists also found that a single dose of the COVID-19 vaccine provided robust protective effects against COVID-19 mortality in the older age group.

Strengths and Limits of the Study

The main strength of this study is the use of the RDD method to estimate the effectiveness of the COVID-19 vaccine, which minimizes the risk of bias. The RDD approach is useful when analyzing cases that involve many confounders while reducing the impact of temporal and geographical differences. Another strength of this study was the fact that approximately 93.9% of the English population aged 75 to 84 had received the first dose of a COVID-19 vaccine.

A major caveat to this study is that it only estimated the effectiveness of the first dose of the COVID-19 vaccine. This method also did not take into account any immune responses that might have occurred due to a previous COVID-19 infection.

*Important NOTE

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

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