Study shows varying population immunity against SARS-CoV-2

By Neha MathurReviewed by Aimee MolineuxDec 30 2021

In a recent study posted to the medRxiv* pre-print server, a team of researchers estimated cumulative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and cumulative coronavirus disease 2019 (COVID-19) vaccination from January 1, 2020, to October 31, 2021, for all US states and 3,138 counties.

Both previous exposure to SARS-CoV-2 and vaccination against the virus contribute to population-level immunity which in turn determines future transmission and disease burden. The authors of the study presumed that the natural- and vaccine-induced immunity wanes over time based on published evidence although there is still substantial uncertainty about how immunity changes over time.

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

The study 

The researchers analyzed the combined distribution of COVID-19 vaccination and prior SARS-CoV-2 infection in each state and county in the US since the beginning of the COVID-19 epidemic to estimate the changes in the population immunity during this period.

They modeled estimates relying on multiple publicly available data sources, such as Merritt et al., which addressed known biases in the Centers for Disease Control and Prevention (CDC) vaccination data. To estimate infection rates, they used a model from Johns Hopkins University, which leveraged case notification and COVID-19 mortality data incorporating uncertainty of their estimates into the model used in this study.

In sensitivity analyses, they examined optimistic and pessimistic scenarios for how protection against infection and severe disease wanes over time, providing a range of possible levels of effective protection.

Results

By October 31, 2021, 56.2% of the US population had prior immunological exposure to SARS-CoV-2 via infection, with state-level estimates ranging from 23% to 72.7% and county-level estimates ranging from 7.0% to 87.5%. More than half of the US population (62.4%) had received at least one COVID-19 vaccine dose, with state-level coverage varying between 25.9 to 78.8% and county-level coverage varying between 7.6% and 90.0%. 

By October 31, 2021, the estimated percentage of the US population with a history of SARS-CoV-2 infection or vaccination (immunologically exposed) was 86.2%, compared to 24.9% on January 1, 2021. State-level estimates for October 31, 2021, ranged between 72.2% and 92.3%. The estimated percentage of the US population with effective protection against COVID-19 infection increased from 14.7% to 49.9% between January 1, 2021, and October 31, 2021, ranging between 37.2% and 59.5%. By this date, 77.4% of the U.S. population was effectively protected against severe disease and ranged between 62.9% and 83.8%.

There were differences in the overall level of immunity achieved between the US states and counties. The relative contribution of vaccination and prior infection to reaching estimated population immunity levels also varied.

The findings of the two additional sensitivity analyses evaluating the sensitivity of effectively protected estimates to the specification of the waning curves showed two possible scenarios. In the optimistic immunity waning scenario, nationally, the population with effective protection against infection by October 31, 2021, was 64.2%, and for those with effective protection against severe disease, it was 85.3%. In the pessimistic scenario, the population with effective protection against infection was 36.6% nationally, and against the severe disease it was 59.3%. 

Limitations and conclusion 

One of the limitations of the study is that it does not take into account the differences in waning immunity based on the virus variant distribution and the effect of booster doses. Further, since the study estimates the relationship between prior infection and vaccination status using survey data, there is a possibility of overlap between the vaccinated and previously infected populations. If that is the case, the overall population immunity would be lower than that estimated in this study.

Furthermore, the COVID-19 epidemic and preventive measures against it are continuously and rapidly changing. In the US, children in the age group of 5 to 11 years became eligible for vaccination on November 1, 2021, and boosters became widely available to individuals over 18 years from November 17, 2021. The impact of such new developments, directly or indirectly, impacting the study findings, were not considered.

Overall, the study findings suggest that the fraction of the US population that was ever infected with SARS-CoV-2 and received at least one dose of a COVID-19 vaccine varied between counties and states.

The study also accounted for the waning of population immunity and showed that a substantial proportion of the US population remains susceptible to SARS-CoV-2 infections.

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