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

Abstract

Background Both SARS-CoV-2 infection and COVID-19 vaccination contribute to population-level immunity against SARS-CoV-2. This study estimates 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 (i) immunological exposure to SARS-CoV-2 (ever infected with SARS-CoV-2 and/or received one or more doses of a COVID-19 vaccine), (ii) effective protection against infection, and (iii) effective protection against severe disease, for each US state and county from January 1, 2020, to December 1, 2021. Results The estimated percentage of the US population with a history of SARS-CoV-2 infection or vaccination as of December 1, 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 December 1, 2021, was 21.8% (95%CrI: 20.7%-23.4%) nationally and ranged between 14.4% (95%CrI: 13.2%-15.8%, West Virginia) to 26.4% (95%CrI: 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% (95%CrI: 47.3%-60.0%, Vermont) and 65.8% (95%CrI: 64.9%-66.7%, Colorado). Conclusions While over four-fifths of the US population had prior immunological exposure to SARS-CoV-2 via vaccination or infection on December 1, 2021, only a fifth of the population was estimated to have effective protection against infection with the immune-evading Omicron variant.

Changes in population immunity against infection and severe disease from SARS-CoV-2 Omicron variants in the United States between December 2021 and November 2022

“…(pessimistic scenario presented shows that population protection against infection would be only around 40% nationally if prior Omicron infections do not offer additional protection against a new (sub)variant. This value is closer to the 22% population immunity estimated for the (at that time) new Omicron variants in December 202117 , as compared to population immunity values in…”

“…Between December 2, 2021 and November 9, 2022, population protection against an Omicron infection rose from 22% (21%–23%) to 63% (51%–75%) nationally (Figure 3AB, eFigure3, eFigure5). In contrast, population immunity against infection by the then circulating pre-Omicron variants in the fall of 2021 (November 9) was estimated to be 51% (47%–56%) 17 . Most of the additional population immunity accrued over the study period was acquired during the initial Omicron surge, with protection against infection estimated as 57% (47%–69%) on March 3, 2022.…”

“…For each location and each week, we computed the percentage of the population immunologically exposed , defined having received at least one dose of a COVID-19 vaccine, at least one prior SARS-CoV-2 infection, or both, using the cumulative reports of first infections and first vaccine doses. Following Klaassen et al (2022), 17 we operationalized the probability of co-occurrence of infection and vaccination as odds ratios for vaccination among individuals with vs. without prior infection, based on data collected in the CDC Household Pulse Surveys 18 and validated against independent survey estimates 19 .…”

“…Third, the infection fatality ratio has been updated to reflect the decreased mortality risk under the Omicron variants (additional eMethods provided in the Supplement). The model used for this analysis was initialized with immunity estimates as of December 2, 2021, as reported by Klaassen et al (2022) 17 .…”

Evaluating the Performance of Centers for Disease Control and Prevention COVID-19 Community Levels as Leading Indicators of COVID-19 Mortality

“…Finally, setting thresholds is complicated by the evolving relationship between epidemiologic indicators and outcomes ( 15–17 ). In the future, this evolution will continue to be driven in large part by features of new variants ( 18 ) but also by levels of increasing and decreasing immunity from infection and vaccination ( 19 ); development and delivery plans for updated vaccines ( 20 ); availability and uptake of testing and antiviral treatments ( 21 , 22 ); and changing policies and practices on preventive measures, such as masking, testing, and isolation and quarantine ( 23 ). In the context of uncertain and changing conditions, well-defined objectives are particularly important to allow efficient updating of metrics to accommodate this flux.…”