COVID-19–Associated Hospitalizations Among Adults During SARS-CoV-2 Delta and Omicron Variant Predominance, by Race/Ethnicity and Vaccination Status

COVID-NET, 14 States, July 2021-January 2022

Christopher A. Taylor, PhD; Michael Whitaker, MPH; Onika Anglin, MPH; Jennifer Milucky, MSPH; Kadam Patel, MPH; Huong Pham, MPH; Shua J. Chai, MD; Nisha B. Alden, MPH; Kimberly Yousey-Hindes, MPH; Evan J. Anderson, MD; Kenzie Teno, MPH; Libby Reeg, MPH; Kathryn Como-Sabetti, MPH; Molly Bleecker, MA; Grant Barney, MPH; Nancy M. Bennett, MD; Laurie M. Billing, MPH; Melissa Sutton, MD; H. Keipp Talbot, MD; Keegan McCaffrey; Fiona P. Havers, MD

Disclosures

Morbidity and Mortality Weekly Report. 2022;71(12):466-473. 

In This Article

Abstract and Introduction

Introduction

Beginning the week of December 19–25, 2021, the B.1.1.529 (Omicron) variant of SARS-CoV-2 (the virus that causes COVID-19) became the predominant circulating variant in the United States (i.e., accounted for >50% of sequenced isolates).* Information on the impact that booster or additional doses of COVID-19 vaccines have on preventing hospitalizations during Omicron predominance is limited. Data from the COVID-19–Associated Hospitalization Surveillance Network (COVID-NET) were analyzed to compare COVID-19–associated hospitalization rates among adults aged ≥18 years during B.1.617.2 (Delta; July 1–December 18, 2021) and Omicron (December 19, 2021–January 31, 2022) variant predominance, overall and by race/ethnicity and vaccination status. During the Omicron-predominant period, weekly COVID-19–associated hospitalization rates (hospitalizations per 100,000 adults) peaked at 38.4, compared with 15.5 during Delta predominance. Hospitalizations rates increased among all adults irrespective of vaccination status (unvaccinated, primary series only, or primary series plus a booster or additional dose). Hospitalization rates during peak Omicron circulation (January 2022) among unvaccinated adults remained 12 times the rates among vaccinated adults who received booster or additional doses and four times the rates among adults who received a primary series, but no booster or additional dose. The rate among adults who received a primary series, but no booster or additional dose, was three times the rate among adults who received a booster or additional dose. During the Omicron-predominant period, peak hospitalization rates among non-Hispanic Black (Black) adults were nearly four times the rate of non-Hispanic White (White) adults and was the highest rate observed among any racial and ethnic group during the pandemic. Compared with the Delta-predominant period, the proportion of unvaccinated hospitalized Black adults increased during the Omicron-predominant period. All adults should stay up to date[1] with COVID-19 vaccination to reduce their risk for COVID-19–associated hospitalization. Implementing strategies that result in the equitable receipt of COVID-19 vaccinations, through building vaccine confidence, raising awareness of the benefits of vaccination, and removing barriers to vaccination access among persons with disproportionately higher hospitalizations rates from COVID-19, including Black adults, is an urgent public health priority.

COVID-NET conducts population-based surveillance for laboratory-confirmed COVID-19–associated hospitalizations in 99 counties across 14 states.§ COVID-19–associated hospitalizations are those occurring among residents of a predefined surveillance catchment area who have a positive real-time reverse transcription–polymerase chain reaction (RT-PCR) or rapid antigen detection test result for SARS-CoV-2 during hospitalization or the 14 days preceding admission.

This analysis describes weekly hospitalization rates during Delta- and Omicron-predominant periods. Among nonpregnant and pregnant adults aged ≥18 years, hospitalization rates were calculated overall, and by race/ethnicity and COVID-19 vaccination status. Age-adjusted rates were calculated by dividing the number of hospitalized COVID-19 patients by population estimates for race/ethnicity, and vaccination status in the catchment area. Vaccination status (unvaccinated, receipt of a primary series only, or receipt of a primary series plus a booster or additional dose) was determined for individual hospitalized patients and for the catchment population using state immunization information systems data**.[2] Monthly incidence among adults who received booster or additional doses was calculated by summing the total number of COVID-19 patients with booster or additional doses hospitalized over all days of the month and dividing by the sum of adults with booster or additional doses in the underlying population for each day of the month.†† This method was also used for calculations in unvaccinated persons and those who received a primary series but not a booster or additional dose.§§

Using previously described methods,[3] investigators collected clinical data on a representative sample of adult patients (7.9%) hospitalized during July 1, 2021–January 31, 2022, stratified by age and COVID-NET site. Surveillance officers abstracted data on sampled patients from medical charts. Pregnant women were excluded because their reasons for hospital admission[4] might differ from those for nonpregnant persons.

Variances were estimated using Taylor series linearization method. Chi-square tests were used to compare differences between the Delta- and Omicron-predominant periods; p-values <0.05 were considered statistically significant. Percentages presented were weighted to account for the probability of selection for sampled cases.[3] Analyses were conducted using SAS statistical software survey procedures (version 9.4; SAS Institute). This activity was reviewed by CDC and conducted consistent with applicable federal law and CDC policy.¶¶

During the Omicron-predominant period, overall weekly adult hospitalization rates peaked at 38.4 per 100,000, exceeding the previous peak on January 9, 2021 (26.1) and the peak rate during the Delta-predominant period (15.5) (Figure 1). Age-adjusted hospitalization rates among Black adults peaked at 94.7 (January 8, 2022), higher than that among all other racial and ethnic groups, 3.8 times the rate among White adults (24.8) for the same week, and 2.5 times the previous peak (January 16, 2021) among Black adults (37.2). This was the highest age-adjusted weekly rate observed among any racial and ethnic group during the pandemic. During the Omicron-predominant period, hospitalization rates increased among unvaccinated persons and those who completed a primary series, with and without receipt of a booster or additional dose (Figure 2). Weekly rates among unvaccinated adults and adults who received a primary COVID-19 vaccination series with a booster or additional dose peaked at 149.8 (January 8, 2022) and 11.7 (January 22, 2022), respectively. The cumulative monthly age-adjusted hospitalization rate during January 2022 among unvaccinated adults (528.2) was 12 times the rates among those who had received a booster or additional dose (45.0) and four times the rates among adults who received a primary series, but no booster or additional dose (133.5). The rate among adults who received a primary series, but no booster or additional dose (133.5), was three times the rate among adults who received a booster or additional dose (45.0).

Figure 1.

Weekly COVID-19–associated hospitalization rates* among adults aged ≥18 years, by race and ethnicity — COVID-19–Associated Hospitalization Surveillance Network, 14 states, March 2020–January 2022
*Overall rates are unadjusted; rates presented by racial and ethnic group are age-adjusted.
Selected counties in California, Colorado, Connecticut, Georgia, Iowa, Maryland, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Tennessee, and Utah (https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm). Starting the week ending December 4, 2021, Maryland data are not included in weekly rate calculations but are included in previous weeks.

Figure 2.

Weekly age-adjusted rates of COVID-19–associated hospitalizations among adults aged ≥18 years, by vaccination status* — COVID-19–Associated Hospitalization Surveillance Network, 13 states, September 4, 2021–January 29, 2022§
Abbreviation: COVID-NET = COVID-19–Associated Hospitalization Surveillance Network.
*Adults who completed a primary vaccination series were defined as those who had received the second dose of a 2-dose primary vaccination series or a single dose of a 1-dose product ≥14 days before a positive SARS-CoV-2 test associated with their hospitalization but received no booster dose. Adults who received booster doses were classified as those who completed the primary series and received an additional or booster dose on or after August 13, 2021, at any time after completion of the primary series, and ≥14 days before a positive test result for SARS-CoV-2, because COVID-19–associated hospitalizations are a lagging indicator and time passed after receipt of a booster dose has been shown to be associated with reduced rates of COVID-19 infection (https://www.nejm.org/doi/full/10.1056/NEJMoa2114255). Adults with no documented receipt of any COVID-19 vaccine dose before the test date were considered unvaccinated.
Selected counties in California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Tennessee, and Utah (https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm). Iowa does not provide data on vaccination status.
§Starting the week ending December 4, 2021, Maryland data are not included in weekly rate calculations but are included in previous weeks. To ensure stability and reliability of rates by vaccination status, data are presented beginning when 14 days have passed since at least 5% of the population of adults aged ≥18 years in the COVID-NET surveillance catchment area had received an additional or booster dose.

Clinical information was abstracted for 5,681 adults with COVID-19–associated hospitalization during July 1, 2021–January 31, 2022 (Table). Black adults accounted for a higher percentage of hospitalizations during the Omicron-predominant period (26.7%) than during the Delta-predominant period (22.2%, p = 0.05). Among all adults, relative to the Delta-predominant period, COVID-19–related illness was the primary reason for admission for a smaller percentage of hospitalizations (87.5% versus 95.5%, p<0.01), and median length of stay was shorter (4 versus 5 days, p<0.01) during the Omicron-predominant period; during this period, the proportion of patients admitted to an intensive care unit, who received invasive mechanical ventilation, and who died in-hospital decreased significantly (all p<0.01).

Among 829 adults hospitalized during the Omicron-predominant period, 49.4% were unvaccinated, compared with 69.5% during the Delta-predominant period (p<0.01). The proportion of hospitalized adults who received booster or additional doses increased from 1.3% during the Delta-predominant period to 13.4% during the Omicron-predominant period (p<0.01)***; among these, 10.7% were long-term care facility residents and 69.5% had an immunosuppressive condition.††† Black adults accounted for 25.2% of all unvaccinated persons hospitalized during the Delta-predominant period; that proportion increased by 23%, to 31.0% during the Omicron-predominant period. Relative to the Delta-predominant period, the proportion of cases in non-Hispanic Asian or Pacific Islanders also increased, whereas the proportion in all other racial and ethnic groups decreased. The proportion of hospitalized Black adults who received a primary COVID-19 vaccination series with or without a booster or additional dose increased from 4.7% and 14.9%, respectively, during the Delta-predominant period to 14.8% and 25.5%, respectively, during the Omicron-predominant period; Hispanic adults experienced smaller increases.

*https://covid.cdc.gov/covid-data-tracker/#variant-proportions
https://www.cdc.gov/coronavirus/2019-ncov/covid-data/covid-net/purpose-methods.html
§Selected counties in California, Colorado, Connecticut, Georgia, Iowa, Maryland, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Tennessee, and Utah (https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm). Iowa did not provide immunization data but is included in the overall population-based hospitalization rates. Maryland did not contribute data after December 4, 2021, but did contribute data for previous weeks.
Rates cannot be stratified by pregnancy status because the underlying population of pregnant women in the catchment area is unknown. Rates are calculated using the CDC National Center for Health Statistics' vintage 2020 bridged-race postcensal population estimates for the counties included in surveillance. https://www.cdc.gov/nchs/nvss/bridged_race.htm
**https://www.medrxiv.org/content/10.1101/2021.08.27.21262356v1
††On August 13, 2021, CDC's Advisory Committee on Immunization Practices (ACIP) issued the first of several recommendations for additional or booster doses of COVID-19 vaccine. Additional recommendations followed and data availability on booster-dose status varies by age because not all age groups were recommended by ACIP to receive booster doses at the same time. https://www.cdc.gov/mmwr/volumes/70/wr/mm7050e2.htm
§§Adults who completed their primary COVID-19 vaccination series were defined as those who had received the second dose of a 2-dose primary vaccination series or a single dose of a 1-dose primary vaccine product ≥14 days before receipt of a positive SARS-CoV-2 test result associated with their hospitalization but received no additional or booster dose. Adults who received booster doses were classified as those who completed their primary vaccination series and received an additional or booster dose of vaccine on or after August 13, 2021, at any time after the completion of their primary series, and ≥14 days before a positive test result for SARS-CoV-2, because COVID-19–associated hospitalizations are a lagging indicator, and time passed after receipt of a booster dose has been shown to be associated with reduced rates of COVID-19 infection (https://www.nejm.org/doi/full/10.1056/NEJMoa2114255). Monthly incidence is based on SARS-CoV-2 positive test result date or, if not known, hospital admission date. Because the immune status of all patients is not known, an additional dose (recommended for persons with a weakened immune system) cannot be distinguished from a booster dose. This is a relevant consideration because vaccines can be less effective in persons with a weakened immune system. Persons who received only 1 vaccine dose of a 2-dose series ≥14 days before the SARS-CoV-2 test date or had received a single dose of either a 1- or 2-dose vaccination series <14 days before the positive SARS-CoV-2 test result were considered partially vaccinated and were not included in rates by vaccination status. Persons who received no doses of any COVID-19 vaccine were considered unvaccinated. The population of unvaccinated adults is determined by subtracting the number of adults who received any dose of vaccine, as previously defined, from the population. When possible, CDC associates a person's primary vaccination series and booster dose with that person. However, linking is sometimes not possible because CDC does not receive personally identifiable information about vaccine doses. This can lead to overestimates of first doses and underestimates of subsequent doses, and underestimates of hospitalization rates in persons who received additional or booster doses. A continuity correction has been applied to the denominators by capping the percent population vaccination coverage at 95% by assuming that at least 5% of each age group would always be unvaccinated in each jurisdiction. This correction ensures that there is always a reasonable denominator for the unvaccinated population that would prevent hospitalization rates from growing unrealistically large because of potential overestimates of vaccination coverage. To ensure stability and reliability of rates by vaccination status, data are presented beginning 14 days after at least 5% of the age group-specific population of the COVID-NET surveillance catchment area has received an additional or booster dose. Additional COVID-NET methods for determining vaccination status have been described previously. https://medrxiv.org/cgi/content/short/2021.08.27.21262356v1
¶¶45 C.F.R. part 46, 21 C.F.R. part 56; 42 U.S.C. Sect. 241(d); 5 U.S.C. Sect. 552a; 44 U.S.C. Sect. 3501 et seq.
***An additional 172 (3.4%, 95% CI = 2.7%–4.2%) adults were partially vaccinated, 69 (0.9%, 95% CI = 0.6–1.2) received a primary vaccination series <14 days before receiving a positive SARS-CoV-2 test result, and 186 (4.1%) had unknown vaccination status; these groups are not further described in this analysis.
†††Includes current treatment or recent diagnosis within the previous 12 months of an immunosuppressive condition or use of an immunosuppressive therapy.

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