The coronavirus disease 2019 (COVID-19) spread worldwide, causing a global health and economic crisis. While many countries are beginning to control the disease using mass vaccination programs, COVID-19 still poses a significant threat.
Study: Estimates of presumed population immunity to SARS-CoV-2 by state in the United States, August 2021. Image Credit: Gargantiopa/ Shutterstock
Growing vaccine hesitancy has been shown to slow the creation of herd immunity, and strains such as the Delta variant appear to be able to evade some level of both natural and vaccine-induced immunity.
Researchers from RTI International have investigated publicly available data published by the Centers for Disease Control and Prevention (CDC) to investigate the proportion of the US population that has become immunized against the disease.
A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.
Herd or population immunity is characterized by a high enough proportion of a population being immunized against the disease to prevent further transmission - enough that the R0 (measure of the degree to which the pathogen is contagious) is below one. This is important to minimize the risk of disease for immunocompromised people who cannot take traditional vaccines.
Population immunity thresholds are high for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) - somewhere between 50% and 83% - and even higher for variants such as the Delta strain, which is likely between 80% and 90%. Knowing the current level of immunity in the population is important for public health policy, especially for removing restrictions that help keep transmission down.
To calculate the immunity present in the population of each state, the researchers calculated the population likely immune due to infection - taking into account reports of seroprevalence that indicate that a proportion of infected individuals do not develop immunity.
The data for seroprevalence per state is provided by the CDC from blood samples from tests unrelated to COVID-19. The researchers only used estimates from tests that use the nucleocapsid protein as the target. While this does allow for a comprehensive picture of immunity from infection, it also massively decreases the number of potential data points as the vast majority of tests look for antibodies that target the receptor-binding domain of the SARS-CoV-2 spike protein.
Following this, the scientists calculated the percentage of individuals who had been fully vaccinated and those that had been partially vaccinated. This was taken from estimates reported by the CDC. To calculate the percentage immunity through vaccination, they multiplied the population who had received a vaccine by the vaccine perceived protection. If unknown, they assumed the vaccine was 85% effective. Vaccine efficacy in this manner does not necessarily mean 85% of people will be immune, and 15% will not be - it is more a measure of the host's ability to fight the virus once vaccinated.
These calculations allowed them to split the population into four categories: immune due to vaccination and infection, immune due to infection, immune due to vaccination, and not immune. For certain analyses, these categories were split into 'immune' and 'not immune.'
The results for each state were then compared against population immunity thresholds. The researchers then performed a sensitivity analysis to investigate the effect of loss of immunity over time. Their final estimates were compared against the states' own estimates.
Across the USA, they found a wide range of results, both in previously infected and currently immune. Generally, high immunity levels were associated with either high vaccination rates or high previous infection levels, followed by moderately successful vaccination programs.
Many states show low overall immunity, with four states showing over 50% of individuals remaining at risk of contracting the disease and a further 19 states hovering near 40%.
This data could be important for informing public health policy, especially the establishment of/removal of restrictions designed to fight disease transmission.
Another key point the authors highlight is the vaccination of children, which generally remained low. Schools potentially provide potent grounds for COVID-19 to spread, so achieving the high population immunity levels required to prevent transmission amongst under 18-year-olds could be paramount.
Figure 3. Percentage of the population vaccinated for COVID-19 and/or previously SARS-CoV-2 infected by state as of August 26, 2021.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information