A new study supports the role of vaccination in reducing the rate of symptomatic coronavirus disease 2019 (COVID-19), and therefore reducing hospitalizations, in over a hundred of the most populated counties in the USA.
A preprint version of the study is available on the medRxiv * server while the article undergoes peer review.
Study: COVID-19 Incidence and Hospitalization Rates are Inversely Related to Vaccination Coverage Among the 112 Most Populous Counties in the United States. Image Credit: Mongkolchon Akesin/ Shutterstock
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
Background
The current surge of the novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the USA is driven by the new delta variant B.1.617.2.
A striking pattern has emerged, with several scientists pointing out that the delta-fueled rise in cases of COVID-19 is localized to places with low vaccination rates. Conversely, the number of infections by this variant also appears to have peaked in places with a high vaccination rate.
How was the study done?
The focus of the current study was on the association between the incidence of infections and the number of hospitalizations for two weeks, up to August 12, compared to the proportion of the population that was completely vaccinated by July 15, 2021.
To achieve a more uniform comparison, the top 112 counties in terms of population (all had over 600,000 inhabitants) were selected, with a total population of 147 million.
The data on incidence and hospitalizations came from the COVID-19 Community Profile Report. The numbers of confirmed hospitalizations due to COVID-19, as recorded on August 12, 2021, for the previous two seven-day spans were first obtained and converted to a 14-day cumulative incidence.
Similarly, the data was used to find the 14-day hospital admission rates specific for COVID-19 per 100,000 and the percentage of COVID-19 cases that led to hospitalization.
Complete vaccination data was also obtained from the Counties tab of this spreadsheet, except for Texas, which the report did not cover. Therefore, other sources were used for this state.
The counties were classified according to their vaccination coverage. The study sample was divided into the lower and upper half of the distribution of vaccination coverage. The median coverage for all counties was approximately 50%, with a mean coverage of ~43% and 57% for the counties in the upper and lower coverage groups, respectively.
What were the findings?
The mean incidence rates for COVID-19 (per 100,000) were 544 and 281 in the upper and lower half counties, respectively. The hospital admission rates per 100 cases were about 9 and 7, respectively. This shows marked differences in the incidence rates and admission rates, depending on whether the county belonged to the upper or lower half.
The case incidence was thus about 1.9 times as high in the upper half as in the lower. In contrast, the ratio of hospitalized COVID-19 cases was about 2.7. This suggests that cases among the more unvaccinated counties were more likely to be severe.
This conclusion is strengthened by the difference in the percentage of hospital admissions among COVID-19 cases, at approximately 9 and 7, respectively. A scatterplot graph shows an inverse relationship between COVID-19 incidence and vaccination coverage and lower hospital admission rates with increasing vaccination coverage rates.
-6.jpg "Fig. 2. COVID-19 Hospital Admission Rate During 7/30 – 8/12/2021 Versus Vaccination Coverage as of 7/15/2021 in 112 U.S. Counties with Population ≥ 600,000. Hospital admission rate is measured on a logarithmic scale as admissions for confirmed cases of COVID-19 per 100,000 population. Vaccination coverage is measured as percent of population fully vaccinated. Vaccination coverage data for 11 Texas counties as of 7/14/2021. Florida counties highlighted in magenta. Texas counties highlighted in cyan. Size of data point proportional to county population.")
Fig. 2. COVID-19 Hospital Admission Rate During 7/30 – 8/12/2021 Versus Vaccination Coverage as of 7/15/2021 in 112 U.S. Counties with Population ≥ 600,000. Hospital admission rate is measured on a logarithmic scale as admissions for confirmed cases of COVID-19 per 100,000 population. Vaccination coverage is measured as percent of population fully vaccinated. Vaccination coverage data for 11 Texas counties as of 7/14/2021. Florida counties highlighted in magenta. Texas counties highlighted in cyan. Size of data point proportional to county population.
When hospital admissions per 100 cases are compared with vaccination coverage, the inverse relationship is confirmed. When the vaccination coverage was increased by ten percentage points, the COVID-19 incidence decreased by over 28%, compared to a 44% drop in hospitalization rates and a 17% reduction in the percentage of cases that required hospitalization.
This data was not adjusted for age-specific risk because of the lack of such information on the report. Along with ethnic and environmental factors and population density, which are likely to be significantly different in different counties, age influences morbidity and severity of illness.
County-wise differences in public health policies such as mask-wearing in public and vaccination mandates could have contributed. Despite the lack of adjustment for these factors, the researchers point out, “the persistence of clearly detectable differences between low- and high-vaccination counties points to an important, identifiable deterrent effect of vaccinations on disease spread.”
What are the implications?
Breakthrough infections are occurring among the fully vaccinated, as is evident from the above data. According to initial reports, these are more likely to be caused by the delta variant, which produces a thousand times as many viral particles as earlier variants. High nasopharyngeal concentrations of the virus are likely to cause transmission even from fully vaccinated individuals infected by the delta variant.
The appearance of data on the reduction in the efficacy of messenger ribonucleic acid (mRNA) vaccines – the Pfizer and Moderna vaccines – among older people such as nursing home residents does not detract from the significant benefit of vaccination on incidence rates of COVID-19 and even more, on disease severity.
This conclusion agrees with a recent paper describing the direction of infections and hospitalizations due to SARS-CoV-2 in New York state.
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
Article Revisions
- Apr 13 2023 - The preprint preliminary research paper that this article was based upon was accepted for publication in a peer-reviewed Scientific Journal. This article was edited accordingly to include a link to the final peer-reviewed paper, now shown in the sources section.
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