SARS-CoV-2 Infection and Hospitalization Among Adults Aged ≥18 Years, by Vaccination Status, Before and During SARS-CoV-2 B.1.1.529 (Omicron) Variant Predominanc

*https://www.medrxiv.org/content/10.1101/2021.12.30.21268495v1
^† https://www.medrxiv.org/content/10.1101/2022.01.07.22268919v2
^§The population of Los Angeles County residents is based on 2019 population estimates prepared for Los Angeles County Internal Services Department. These population estimates exclude the populations of Pasadena and Long Beach, which have independent public health departments.
^¶Vaccination status was determined using a deterministic and probabilistic matching algorithm to link cases with immunization records in CAIR2 and followed CDC guidelines. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/fully-vaccinated-guidance.html
**CDC recommends that persons aged ≥5 years who are moderately or severely immunocompromised receive a third dose of an mRNA COVID-19 vaccine 28 days after completion of the primary series to enhance the likelihood of mounting an adequate immune response. A booster is recommended for persons aged ≥12 years to protect against waning of vaccine-induced immunity. Recommendations for booster timing and vaccine manufacturer differs by age group and vaccine product received during the primary series (https://www.cdc.gov/coronavirus/2019-ncov/vaccines/recommendations/immuno.html). This analysis does not distinguish between fully vaccinated persons who are immunocompromised and received a booster and fully vaccinated persons who are not immunocompromised and received a booster.
^††Partially vaccinated persons were those who had received the first dose in a 2-dose series ≥14 days earlier, but who were either missing a second dose or <14 days had elapsed since receipt of the second dose. Recipients of the Janssen vaccine <14 days earlier were considered unvaccinated. Partially vaccinated persons accounted for 4% of the sample (18,988). It was also not possible to differentiate between persons who received 1 dose in a 2-dose series (Pfizer-BioNTech or Moderna vaccines) and persons who were misclassified because of missing data.
^§§Whole genome sequencing lineage data were from all sequencing results reported to LACDPH or sequenced after specimens were referred to LACDPH laboratories. Additional variant data were available for specimens sequenced for enhanced Omicron variant surveillance. During the Delta predominant period, sequencing data were available for 20%–25% of SARS-CoV-2 specimens. As infections increased during the Omicron predominant period, capacity for whole genome sequencing did not increase proportionally; sequencing data were available for 1%–18% of SARS-CoV-2 specimens.
^¶¶Adjusted rates were calculated using methods documented by CDC (https://www.cdc.gov/cancer/uscs/technical_notes/stat_methods/rates.htm). A continuity correction was applied to estimates of the unvaccinated population to ensure that ≥5% of the population remains unvaccinated using the methods outlined elsewhere (https://kingcounty.gov/depts/health/covid-19/data/~/media/depts/health/communicable-diseases/documents/C19/calculation-method-technical-appendix.ashx). Adjusted rate ratios were calculated as the cumulative incidence among unvaccinated persons divided by the cumulative incidence among those fully vaccinated with or without booster.