Severity of Disease Among Adults Hospitalized with Laboratory-Confirmed COVID-19 Before and During the Period of SARS-CoV-2 B.1.617.2 (Delta) Predominance

COVID-NET, 14 States, January-August 2021

Christopher A. Taylor, PhD; Kadam Patel, MPH; Huong Pham, MPH; Michael Whitaker, MPH; Onika Anglin, MPH; Anita K. Kambhampati, MPH; Jennifer Milucky, MSPH; Shua J. Chai, MD; Pam Daily Kirley, MPH; Nisha B. Alden, MPH; Isaac Armistead, MD; James Meek, MPH; Kimberly Yousey-Hindes, MPH; Evan J. Anderson, MD; Kyle P. Openo, DrPH; Kenzie Teno, MPH; Andy Weigel; Maya L. Monroe, MPH; Patricia A. Ryan, MS; Justin Henderson, MPH; Val Tellez Nunez, MPH; Erica Bye, MPH; Ruth Lynfield, MD; Mayvilynne Poblete, MA, MPH; Chad Smelser, MD; Grant R. Barney, MPH; Nancy L. Spina, MPH; Nancy M. Bennett, MD; Kevin Popham, MPH; Laurie M. Billing, MPH; Eli Shiltz, MPH; Nasreen Abdullah, MD; Melissa Sutton, MD; William Schaffner, MD; H. Keipp Talbot, MD; Jake Ortega, MPH; Andrea Price; Shikha Garg, MD; Fiona P. Havers, MD

Disclosures

Morbidity and Mortality Weekly Report. 2021;70(43):1513-1519. 

In This Article

Abstract and Introduction

Introduction

In mid-June 2021, B.1.671.2 (Delta) became the predominant variant of SARS-CoV-2, the virus that causes COVID-19, circulating in the United States. As of July 2021, the Delta variant was responsible for nearly all new SARS-CoV-2 infections in the United States.* The Delta variant is more transmissible than previously circulating SARS-CoV-2 variants;[1] however, whether it causes more severe disease in adults has been uncertain. Data from the CDC COVID-19–Associated Hospitalization Surveillance Network (COVID-NET), a population-based surveillance system for COVID-19–associated hospitalizations, were used to examine trends in severe outcomes in adults aged ≥18 years hospitalized with laboratory-confirmed COVID-19 during periods before (January–June 2021) and during (July–August 2021) Delta variant predominance. COVID-19–associated hospitalization rates among all adults declined during January–June 2021 (pre-Delta period), before increasing during July–August 2021 (Delta period). Among sampled nonpregnant hospitalized COVID-19 patients with completed medical record abstraction and a discharge disposition during the pre-Delta period, the proportion of patients who were admitted to an intensive care unit (ICU), received invasive mechanical ventilation (IMV), or died while hospitalized did not significantly change from the pre-Delta period to the Delta period. The proportion of hospitalized COVID-19 patients who were aged 18–49 years significantly increased, from 24.7% (95% confidence interval [CI] = 23.2%–26.3%) of all hospitalizations in the pre-Delta period, to 35.8% (95% CI = 32.1%–39.5%, p<0.01) during the Delta period. When examined by vaccination status, 71.8% of COVID-19–associated hospitalizations in the Delta period were in unvaccinated adults. Adults aged 18–49 years accounted for 43.6% (95% CI = 39.1%–48.2%) of all hospitalizations among unvaccinated adults during the Delta period. No difference was observed in ICU admission, receipt of IMV, or in-hospital death among nonpregnant hospitalized adults between the pre-Delta and Delta periods. However, the proportion of unvaccinated adults aged 18–49 years hospitalized with COVID-19 has increased as the Delta variant has become more predominant. Lower vaccination coverage in this age group likely contributed to the increase in hospitalized patients during the Delta period. COVID-19 vaccination is critical for all eligible adults, including those aged <50 years who have relatively low vaccination rates compared with older adults.

COVID-NET conducts population-based surveillance for laboratory-confirmed COVID-19–associated hospitalizations in 99 counties across 14 states. Among residents of a predefined surveillance catchment area, COVID-19–associated hospitalizations are defined as a positive real-time reverse transcription–polymerase chain reaction or rapid antigen detection test result for SARS-CoV-2 during hospitalization or within the 14 days preceding admission.§ Unadjusted age-specific monthly population-based hospitalization rates (hospitalizations per 100,000 persons) among all adults aged ≥18 years irrespective of pregnancy status during January–August 2021 were calculated by dividing the total number of hospitalized COVID-19 patients by population estimates within each age group in the surveillance catchment area. Using previously described methods,[2] clinical outcomes data were collected on a representative sample of hospitalized adults stratified by age and site of admission during January–August 2021. Using a standardized case report form, trained surveillance staff members abstracted data on sampled cases (updated monthly) from medical charts that included a discharge disposition. Pregnant women (496) were excluded from the analysis because reasons for hospital admission[3] and standards for ICU admission might differ from those for nonpregnant persons. Severe outcomes assessed included ICU admission, receipt of IMV, and all cause in-hospital death. Severe outcomes were compared during periods before (pre-Delta period) and during Delta variant predominance (Delta period). Because COVID-19 vaccination might affect clinical outcomes,[4] and vaccination coverage changed during the study period, results were analyzed overall and stratified by COVID-19 vaccination status.** Vaccination status was determined using state immunization information systems data.[5,6] Variances were estimated using Taylor series linearization method. Chi-square testing was used to compare differences between the pre-Delta and Delta periods; p-values <0.05 were considered statistically significant, adjusted for multiple comparisons using the Bonferroni correction method. Unless otherwise noted, percentages presented are weighted to account for the probability of selection for sampled cases.[2] All analyses were conducted using SAS statistical software survey procedures (version 9.4; SAS Institute). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.††

Based on 87,879 COVID-19 hospitalizations among all adults during January 1–August 31, 2021, irrespective of pregnancy status, monthly population-based rates of COVID-19–associated hospitalizations declined among all adult age groups during the pre-Delta period (Figure 1). Rates subsequently increased during July–August, with the highest rates among adults aged ≥65 years and the lowest among those aged 18–49 years. Monthly ICU admission, IMV, and in-hospital death rates followed the same patterns as COVID-19–associated hospitalization rates by age group, with the highest rates in adults aged ≥65 years and the lowest in persons aged 18–49 years.

Figure 1.

COVID-19–associated monthly hospitalization rates per 100,000 population among adults aged ≥18 years,* by age group, month, and period relative to SARS-CoV-2 B.1.617.2 (Delta) variant predominance — COVID-NET, 14 states,§ January–August 2021
*Proportions are from a weighted sample of hospitalized adults with completed medical chart abstraction and a discharge disposition. Results are subject to change as additional data are reported.
January–June 2021 is the pre-Delta period; the Delta period (July–August 2021) is when the Delta variant 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 can be found at https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm.

During January–August 2021, in a representative sample of 7,615 COVID-19 hospitalizations among nonpregnant adults with detailed clinical data available, 71.8% (weighted) of patients hospitalized during the Delta period were unvaccinated. Among unvaccinated hospitalized COVID-19 patients, the average monthly proportion who were aged 18–49 years significantly increased from 26.9% in the pre-Delta period to 43.6% during the Delta period (p<0.01) (Table). Among hospitalized COVID-19 patients who were fully vaccinated, the proportion of younger adults did not significantly change between the pre-Delta (10.6%) and Delta (10.8%) periods. Among sampled nonpregnant adults hospitalized with COVID-19, no statistically significant differences were observed between the pre-Delta and Delta periods by sex, race/ethnicity, or the proportion of patients who were admitted to an ICU, who received IMV, or who died while hospitalized, overall and stratified by age and vaccination status.

During January–August 2021, the proportion of patients aged ≥50 years hospitalized with COVID-19 who were admitted to an ICU or who died while hospitalized generally trended upward in the Delta period (Figure 2), with the largest increase in persons who died while hospitalized among adults aged ≥65 years, (from 10.2% in June to 18.1% in August), although the difference was not statistically significant (p = 0.70). Monthly proportions of adults hospitalized with COVID-19 who received IMV also did not change significantly during January–August 2021.

Figure 2.

Percentage* of nonpregnant adult patients hospitalized with COVID-19 who were admitted to an intensive care unit and who died while hospitalized, by age group, month, and period relative to SARS-CoV-2 B.1.617.2 (Delta) variant predominance — COVID-NET, 14 states,§ January–August 2021
*Proportions are from a weighted sample of hospitalized adults with completed medical chart abstraction and a discharge disposition. Results are subject to change as additional data are reported.
January–June 2021 is the pre-Delta period; the Delta period (July–August 2021) is when the Delta variant 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 can be found at https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm.

*https://covid.cdc.gov/covid-data-tracker/#variant-proportions
Selected counties in California, Colorado, Connecticut, Georgia, Iowa, Maryland, Michigan, Minnesota, New Mexico, New York, Ohio, Oregon, Tennessee, and Utah can be found at https://www.cdc.gov/mmwr/volumes/69/wr/mm6915e3.htm.
§ https://www.cdc.gov/coronavirus/2019-ncov/covid-data/covid-net/purpose-methods.html
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 National Center for Health Statistics' vintage 2019 bridged-race postcensal population estimates for the counties included in surveillance. https://www.cdc.gov/nchs/nvss/bridged_race.htm
**Fully vaccinated adults with a COVID-19–associated hospitalization were persons who had received the second dose of a 2-dose COVID-19 vaccine series or a single dose of a 1-dose product ≥14 days before receiving a positive SARS-CoV-2 test result associated with their hospitalization. Adults whose positive 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 vaccine <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 missing vaccination status and were included in total proportions not stratified by vaccination status.
††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.

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