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. 2023 Feb 8;76(3):e350-e359.
doi: 10.1093/cid/ciac438.

Population Immunity to Pre-Omicron and Omicron Severe Acute Respiratory Syndrome Coronavirus 2 Variants in US States and Counties Through 1 December 2021

Fayette Klaassen  1 , Melanie H Chitwood  2 , Ted Cohen  2 , Virginia E Pitzer  2 , Marcus Russi  2 , Nicole A Swartwood  1 , Joshua A Salomon  3 , Nicolas A Menzies  1
Affiliations

Population Immunity to Pre-Omicron and Omicron Severe Acute Respiratory Syndrome Coronavirus 2 Variants in US States and Counties Through 1 December 2021

Fayette Klaassen et al. Clin Infect Dis. .

Abstract

Background: Both severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (COVID-19) vaccination contribute to population-level immunity against SARS-CoV-2. This study estimated the immunological exposure and effective protection against future SARS-CoV-2 infection in each US state and county over 2020-2021 and how this changed with the introduction of the Omicron variant.

Methods: We used a Bayesian model to synthesize estimates of daily SARS-CoV-2 infections, vaccination data and estimates of the relative rates of vaccination conditional on infection status to estimate the fraction of the population with (1) immunological exposure to SARS-CoV-2 (ever infected with SARS-CoV-2 and/or received ≥1 doses of a COVID-19 vaccine), (2) effective protection against infection, and (3) effective protection against severe disease, for each US state and county from 1 January 2020 to 1 December 2021.

Results: The estimated percentage of the US population with a history of SARS-CoV-2 infection or vaccination as of 1 December 2021 was 88.2% (95% credible interval [CrI], 83.6%-93.5%). Accounting for waning and immune escape, effective protection against the Omicron variant on 1 December 2021 was 21.8% (95% CrI, 20.7%-23.4%) nationally and ranged between 14.4% (13.2%-15.8%; West Virginia) and 26.4% (25.3%-27.8%; Colorado). Effective protection against severe disease from Omicron was 61.2% (95% CrI, 59.1%-64.0%) nationally and ranged between 53.0% (47.3%-60.0%; Vermont) and 65.8% (64.9%-66.7%; Colorado).

Conclusions: While more than four-fifths of the US population had prior immunological exposure to SARS-CoV-2 via vaccination or infection on 1 December 2021, only a fifth of the population was estimated to have effective protection against infection with the immune-evading Omicron variant.

Keywords: SARS-CoV-2; effective protection‌; immunological exposure.

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Conflict of interest statement

Potential conflicts of interest . M. H. C. reports support for attending meetings and/or travel from the World Health Organization Global Task Force on TB Impact Measurement. V. E. P. reports grants to her institution, unrelated to this work, from the Bill & Melinda Gates Foundation, the Centers for Disease Control and Prevention, Gavi, the Vaccine Alliance, and the National Institute of Allergy and Infectious Diseases, National Institutes of Health; is an unpaid member of the World Health Organization’s Immunization and Vaccines Related Implementation Research Advisory Committee; and has received reimbursement from Merck and Pfizer for travel expenses to Scientific Input Engagements unrelated to the current work (<$500). N. A. S. reports grants or contracts, unrelated to the current work, from the Centers for Disease Control and Prevention and the National Institutes of Health. N. A. M. reports grants or contracts to their institution, unrelated to the current work from Bill & Melinda Gates Foundation, the European Commission, the World Health Organization, and USAID; consulting fees paid to the author from the World Health Organization and the Global Fund to Fight AIDS TB and Malaria; and support for attending meetings and/or travel from the Bill & Melinda Gates Foundation. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

Figures

Figure 1.
Figure 1.
Estimated percentage immunologically exposed on 1 December 2021, for each US county and state. Background coloring indicates the state-specific distribution of immunity as a function of infections and vaccinations. Black dots represent counties in a state; red dots, the state average. Orange diamonds represent the state population–weighted national averages of the percentage ever infected and vaccinated (this does not represent the national average of immunity because the calculation for immunity is state specific).
Figure 2.
Figure 2.
State-level estimates and uncertainty intervals of the percentages immunologically exposed, effectively protected against infection, and effectively protected against severe disease over time, with estimates of effective protection against Omicron infection under 3 immune-escape scenarios.
Figure 3.
Figure 3.
County-level estimates of the percentage of the population effectively protected against infection at 4 time points between 1 January and 1 December 2021. Counties excluded owing to missing or insufficient data are colored yellow.
Figure 4.
Figure 4.
Relative contributions of prior infection and vaccination to population immunity for each county at 4 time points between 1 January and 1 December 2021. The percentage ever infected and the percentage vaccinated are categorized with cutoff scores of 40%, 50%, and 60%, values that roughly correspond to the quantile breakpoints of the estimates of those ever infected and vaccinated on 1 December 2021.
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References

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