Using data representing 80% of the US population, the researchers found that increased vaccination coverage against coronavirus disease 2019 (COVID-19) may be linked to reduced incidence of cases and deaths. The study was published in the British medical journal (BMJ) and is available for free.

Study: Public health impact of covid-19 vaccines in the United States: observational study. Image Credit: Prostock-studio/Shutterstock


A year after the start of the COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the first vaccines based on messenger RNA (mRNA) technology were administered under emergency use authorization in December 2020.

Despite the exceptional efficacy of vaccines in clinical trials, real-world situations may be different due to a plethora of practical challenges (such as maintaining the cold chain when scaling up immunization programs, logistical challenges during mass vaccination and accurate reporting of health outcomes).

Similarly, individual effects on disease risk and progression may also be complemented by secondary benefits of vaccination, such as slowing viral spread in the community and hindering transmission (with implications in downstream for morbidity and mortality).

However, so far, population-level data has been limited, which has yielded rather limited information on COVID-19 vaccine effectiveness at scale. One reason is that reported cases may not always truly represent transmission rates due to the noticeable variation in individual tests.

This recent, large and rigorous observational study – led by researchers at the Centers for Disease Control and Prevention (CDC) in Atlanta, USA – used national county-level surveillance data to answer a simple question : what are the benefits of the COVID-19 vaccine in the real world?

Using big data to count cases and deaths

In this study, the number of cases and deaths were disaggregated by county and time using the CDC’s case surveillance dataset. County COVID-19 death rates and disease incidence were the primary and secondary outcomes of the study, respectively.

Additionally, the researchers used incidence rate ratios to compare rates between levels of vaccine coverage and estimated the impact of a 10% improvement in county vaccine coverage. The latter was defined as at least one dose of a COVID-19 vaccine in adults 18 years of age or older.

It should be emphasized that the study was conducted when the Alpha (B.1.1.7) and Delta (B.1.617.2) variants of SARS-CoV-2 predominated in the population. The study compared the impact of very low (0-9%), low (10-39%), medium (40-69%) and high (more than 70%) vaccination coverage rates.

Drop in COVID-19 rates

Researchers have demonstrated that a 10% improvement in vaccination coverage can be linked to an 8% reduction in death rates from COVID-19, as well as a 7% reduction in the incidence of the disease – two very significant percentage decreases at the population level.

The study had also found downward trends in deaths and case incidence when higher levels of vaccination were applied during the predominance of Alpha and Delta variants of SARS-CoV-2. This effect did not change when different sensitivity analyzes were applied, improving confidence in these results and predictive capabilities.

Given some limitations of the study, including additional unexplored markers of disease severity (eg, hospital admissions) and lack of control for physical distancing, masking, or other variables potential confounders, the results of the study can be considered quite robust.

Consideration of impact at the population level

Even though the study period did not cover the current predominance of the Omicron SARS-CoV-2 variant, this article has shown how reduced vaccine efficacy and the importance of staying current with COVID-19 vaccinations could translate into changes in the vaccine at the population level impact.

Because community benefits are rooted in individual benefits, for which vaccine effectiveness has been established in countries around the world, these data may be generalizable to other countries,” the study authors state. published in the British medical journal (BMJ).

Future research could benefit from assessing the macroeconomic effects of improving population health, such as changes in employment rates and gross domestic product resulting from the reopening of society,” they point out. .

The decline in incidence that goes hand in hand with the increase in vaccination coverage is rather consistent with ongoing surveillance data elsewhere. Therefore, the continued strategic deployment of COVID-19 vaccines must be complemented by public health and social measures in ongoing viral transmission.


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