Population immunity to pre-Omicron and Omicron SARS-CoV-2 variants in US states and counties through December 1, 2021

Prior infection and vaccination both contribute to population-level SARS-CoV-2 immunity. We used a Bayesian model to synthesize evidence and estimate population immunity to prevalent SARS-CoV-2 variants in the United States over the course of the epidemic until December 1, 2021, and how this changed with the introduction of the Omicron variant. We used daily SARS-CoV-2 infection estimates and vaccination coverage data for each US state and county. We estimated relative rates of vaccination conditional on previous infection status using the Census Bureau’s Household Pulse Survey. We used published evidence on natural and vaccine-induced immunity, including waning and immune escape. 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%CrI: 83.6%−93.5%), compared to 24.9% (95%CrI: 18.5%−34.1%) on January 1, 2021. State-level estimates for December 1, 2021, ranged between 76.9% (95%CrI: 67.6%−87.6%, West Virginia) and 94.4% (95%CrI: 91.2%−97.3%, New Mexico). Accounting for waning and immune escape, the 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). While over three-quarters 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 to infection with the immune-evading Omicron variant.