Higher immunization coverage associated with lower mortality and incidence rates of COVID-19 at the population level in the United States

Using data representing 80% of the US population, researchers have found that increased immunization coverage against coronavirus disease 2019 (COVID-19) may be associated with a lower incidence of cases and deaths. The study was published in the journal British Medical Journal (BMJ) and is available free of charge.

Study: Public Health Impact of Covid-19 Vaccines in the United States: Observational Study. Credit: Prostock Studio/Shutterstock


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

Notwithstanding the exceptional efficacy of vaccines in clinical trials, real-world situations may differ due to a variety of practical challenges (e.g., maintaining cold chains when expanding immunization programs, logistical challenges in mass vaccination, and accurate reporting of health outcomes).

Similarly, individual effects on disease risk and progression can also be complemented by secondary benefits of vaccination, e.g. B. slowing the spread of the virus in the community and preventing onward transmission (with downstream effects on morbidity and mortality).

So far, however, population-level data has been limited, providing a rather limited view of the large-scale effectiveness of the COVID-19 vaccine. One reason is that reported cases don’t always really represent transmission rates due to the noticeable variability in individual tests.

This recent large and rigorously 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 COVID-19? vaccine in the real world?

Using big data to count cases and deaths

In this study, the number of cases and deaths was broken down by county and time period using the CDC’s case-monitoring dataset. The county’s COVID-19 mortality rates and disease incidence rates were the primary and secondary findings of the study, respectively.

In addition, the researchers used incidence rate ratios to compare rates between immunization rates and estimated the impact of a 10 percent improvement in immunization rates across counties. The latter was defined as giving at least one dose of the COVID-19 vaccine to adults aged 18 and over.

It must be emphasized that the study was conducted when SARS-CoV-2 variants Alpha (B.1.1.7) and Delta (B.1.617.2) were prevalent in the population. The study compared the effects of very low (0-9%), low (10-39%), medium (40-69%) and high (over 70%) vaccination rates.

Lower rates of COVID-19

Researchers have shown that a 10 percent improvement in immunization coverage can be associated with an 8 percent reduction in death rates from COVID-19, as well as a 7 percent reduction in the incidence of the disease — two very significant percentage decreases in the population stage.

The study had also shown downward trends in deaths and case incidence when higher levels of vaccination were used during the dominance 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 ability.

Given some limitations of the study, including additional unexplored markers of disease severity (e.g., hospitalizations) and the lack of controls for physical distancing, masking, or other potential confounding variables, the study results can be considered quite robust.

Consider the impact at the population level

Although the study period did not cover the current dominance of the omicron variant of SARS-CoV-2, this article demonstrated how the reduction in vaccine effectiveness and the importance of staying current with immunizations can lead to changes in vaccine impacts at the population level.

Because the community benefits are based on individual benefits for which vaccine efficacy has been established in countries around the world, these data can be generalized to other countries,” the authors explain in this study, published in British Medical Journal (BMJ).

Future research could benefit from assessing the macroeconomic impact 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 has accompanied the increase in immunization coverage is fairly consistent with ongoing surveillance data elsewhere. Therefore, the continued strategic use of COVID-19 vaccines should be complemented by public health and social measures related to the ongoing virus transmission.

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