Close

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).

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.

Acknowledgments

¹CDC COVID-19 Emergency Response Team; ²General Dynamics Information Technology, Atlanta, Georgia; ³California Emerging Infections Program, Oakland, California; ⁴Career Epidemiology Field Officer Program, CDC; ⁵Colorado Department of Public Health & Environment; ⁶Connecticut Emerging Infections Program, Yale School of Public Health, New Haven, Connecticut; ⁷Emory University School of Medicine, Atlanta, Georgia; ⁸Georgia Emerging Infections Program, Georgia Department of Public Health; ⁹Atlanta Veterans Affairs Medical Center, Atlanta, Georgia; ¹⁰Iowa Department of Public Health; ¹¹Michigan Department of Health and Human Services; ¹²Minnesota Department of Health; ¹³New Mexico Emerging Infections Program, University of New Mexico, Albuquerque, New Mexico; ¹⁴New York State Department of Health; ¹⁵University of Rochester School of Medicine and Dentistry, Rochester, New York; ¹⁶Ohio Department of Health; ¹⁷Public Health Division, Oregon Health Authority; ¹⁸Vanderbilt University Medical Center, Nashville, Tennessee; ¹⁹Utah Department of Health.

References

1. CDC. Stay up to date with your COVID-19 vaccines. Atlanta, GA: US Department of Health and Human Services, CDC; 2022. Accessed March 10, 2022. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/stay-up-to-date.html
2. Moline HL, Whitaker M, Deng L, et al. Effectiveness of COVID-19 vaccines in preventing hospitalization among adults aged ≥65 years—COVID-NET, 13 states, February–April 2021. MMWR Morb Mortal Wkly Rep 2021;70:1088–93. https://doi.org/10.15585/mmwr.mm7032e3external icon PMID:34383730external icon
3. Garg S, Patel K, Pham H, et al. Clinical trends among U.S. adults hospitalized with COVID-19, March to December 2020: a cross-sectional study. Ann Intern Med 2021;174:1409–19. https://doi.org/10.7326/M21-1991external icon PMID:34370517external icon
4. Delahoy MJ, Whitaker M, O’Halloran A, et al.; COVID-NET Surveillance Team. Characteristics and maternal and birth outcomes of hospitalized pregnant women with laboratory-confirmed COVID-19—COVID-NET, 13 states, March 1–August 22, 2020. MMWR Morb Mortal Wkly Rep 2020;69:1347–54. https://doi.org/10.15585/mmwr.mm6938e1external icon PMID:32970655external icon
5. Johnson AG, Amin AB, Ali AR, et al. COVID-19 incidence and death rates among unvaccinated and fully vaccinated adults with and without booster doses during periods of Delta and Omicron variant emergence—25 U.S. Jurisdictions, April 4–December 25, 2021. MMWR Morb Mortal Wkly Rep 2022;71:132–8. https://doi.org/10.15585/mmwr.mm7104e2external icon PMID:35085223external icon
6. Iuliano AD, Brunkard JM, Boehmer TK, et al. Trends in disease severity and health care utilization during the early Omicron variant period compared with previous SARS-CoV-2 high transmission periods—United States, December 2020–January 2022. MMWR Morb Mortal Wkly Rep 2022;71:146–52. https://doi.org/10.15585/mmwr.mm7104e4external icon PMID:35085225external icon
7. Acosta AM, Garg S, Pham H, et al. Racial and ethnic disparities in rates of COVID-19–associated hospitalization, intensive care unit admission, and in-hospital death in the United States from March 2020 to February 2021. JAMA Netw Open 2021;4:e2130479. https://doi.org/10.1001/jamanetworkopen.2021.30479external icon PMID:34673962external icon

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/NEJMoa2114255external icon). 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.

Abbreviations: COVID-NET = COVID-19–Associated Hospitalization Surveillance Network; ICU = intensive care unit; IMV = invasive mechanical ventilation; LTCF = long-term care facility; NA = not applicable.
* Data are from a weighted sample of hospitalized nonpregnant adults with completed medical record abstractions and a discharge disposition. Sample sizes presented are unweighted with weighted percentages.
^† Vaccination status is based on state immunization information system data. Adults who completed a primary vaccination series were persons who had received the second dose of a 2-dose COVID-19 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 or additional 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, as 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/NEJMoa2114255external icon). Adults with a positive result whose SARS-CoV-2 test date was ≥14 days after the first dose of a 2-dose series but <14 days after receipt of the second dose were considered partially vaccinated. Partially vaccinated adults, and those who received a single dose of a 1-dose product <14 days before the positive SARS-CoV-2 test result were not included in analyses by vaccination status but were included in rates and overall proportions that were not stratified by vaccination status. Adults with no documented receipt of any COVID-19 vaccine dose before the test date were considered unvaccinated. If the SARS-CoV-2 test date was not available, hospital admission date was used. Adults whose vaccination status had not yet been verified using the immunization information system data were considered to have unknown vaccination status and were included in total proportions but not stratified by vaccination status. Vaccination status is not available for Iowa and cases from Iowa are excluded from analyses that examined vaccination status. Additional COVID-NET methods for determining vaccination status have been described previously. https://www.medrxiv.org/content/10.1101/2021.08.27.21262356v1external icon
^§ Delta period: July 1, 2021–December 18, 2021, reflects the time when Delta was the predominant circulating variant; Omicron period: December 19, 2021–January 31, 2022, reflects the time when Omicron was the predominant circulating variant.
^¶ 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 does not provide data on vaccination status. Starting the week ending December 4, 2021, Maryland data are not included in calculations but are included in previous weeks.
** Total hospitalizations include data from selected counties in 14 COVID-NET states irrespective of vaccination status and includes adults with partial or unknown vaccination status. As a result, the number of total hospitalizations exceeds the sum of unvaccinated adults, adults who received a primary series without a booster or additional dose, and adults who received a primary series with a booster or additional dose.
^†† Proportions between the pre-Delta and Delta period were compared using chi-square tests; p-values <0.05 were considered statistically significant, adjusted for multiple comparisons using the Bonferroni correction method.
^§§ Percentages presented for the overall number are weighted row percentages. Percentages presented for demographic characteristics are weighted column percentages.
^¶¶ If ethnicity was unknown, non-Hispanic ethnicity was assumed. Persons with multiple, unknown, or missing race accounted for 6.9% (weighted) of all cases. These persons are excluded from the proportions of race/ethnicity but are included in other analyses.
*** LTCF residents include hospitalized adults who were identified as residents of a nursing home/skilled nursing facility, rehabilitation facility, assisted living/residential care, long-term acute care hospital, group/retirement home, or other LTCF upon hospital admission. A free-text field for other types of residences was examined; patients with an LTCF-type residence were also categorized as LTCF residents.
^††† Defined as one or more of the following: chronic lung disease including asthma, chronic metabolic disease including diabetes mellitus, blood disorder/hemoglobinopathy, cardiovascular disease, neurologic disorder, immunocompromising condition, renal disease, gastrointestinal/liver disease, rheumatologic/autoimmune/inflammatory condition, obesity, feeding tube dependency, and wheelchair dependency.
^§§§ Includes current treatment or recent diagnosis of an immunosuppressive condition or use of an immunosuppressive therapy during the preceding 12 months.
^¶¶¶ COVID-19–associated signs and symptoms included respiratory symptoms (congestion or runny nose, cough, hemoptysis or bloody sputum, shortness of breath or respiratory distress, sore throat, upper respiratory infection, influenza-like illness, and wheezing) and non-respiratory symptoms (abdominal pain, altered mental status or confusion, anosmia or decreased smell, chest pain, conjunctivitis, diarrhea, dysgeusia or decreased taste, fatigue, fever or chills, headache, muscle aches or myalgias, nausea or vomiting, rash, and seizures). Symptoms are abstracted from the medical chart and might not be complete.
**** ICU admission and IMV are not mutually exclusive categories, and patients could have received both.
^†††† ICU admission status was missing in 1.3% (weighted) of hospitalizations; these hospitalizations are included in other analyses.
^§§§§ IMV status was missing in 1.4% (weighted) of hospitalizations; these hospitalizations are otherwise included elsewhere in the analysis.
^¶¶¶¶ In-hospital death status was missing in 1.4% (weighted) of hospitalizations; these hospitalizations are otherwise included elsewhere in the analysis.
***** 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 a positive for SARS-CoV-2 test result, and 186 (4.1%) had unknown vaccination status; these groups are not further described in this analysis.
^{†††††} If SARS-CoV-2 test date was missing, hospitalization admission date was used.